diff --git a/changelog.d/11846.feature b/changelog.d/11846.feature
new file mode 100644
index 000000000..fcf6affdb
--- /dev/null
+++ b/changelog.d/11846.feature
@@ -0,0 +1 @@
+Allow configuring a maximum file size as well as a list of allowed content types for avatars.
diff --git a/docs/sample_config.yaml b/docs/sample_config.yaml
index 3e9def56e..9b45e4ddd 100644
--- a/docs/sample_config.yaml
+++ b/docs/sample_config.yaml
@@ -479,73 +479,19 @@ limit_remote_rooms:
 #
 #allow_per_room_profiles: false
 
-# Whether to show the users on this homeserver in the user directory. Defaults to
-# 'true'.
+# The largest allowed file size for a user avatar. Defaults to no restriction.
 #
-#show_users_in_user_directory: false
-
-# Message retention policy at the server level.
+# Note that user avatar changes will not work if this is set without
+# using Synapse's media repository.
 #
-# Room admins and mods can define a retention period for their rooms using the
-# 'm.room.retention' state event, and server admins can cap this period by setting
-# the 'allowed_lifetime_min' and 'allowed_lifetime_max' config options.
+#max_avatar_size: 10M
+
+# The MIME types allowed for user avatars. Defaults to no restriction.
 #
-# If this feature is enabled, Synapse will regularly look for and purge events
-# which are older than the room's maximum retention period. Synapse will also
-# filter events received over federation so that events that should have been
-# purged are ignored and not stored again.
+# Note that user avatar changes will not work if this is set without
+# using Synapse's media repository.
 #
-retention:
-  # The message retention policies feature is disabled by default. Uncomment the
-  # following line to enable it.
-  #
-  #enabled: true
-
-  # Default retention policy. If set, Synapse will apply it to rooms that lack the
-  # 'm.room.retention' state event. Currently, the value of 'min_lifetime' doesn't
-  # matter much because Synapse doesn't take it into account yet.
-  #
-  #default_policy:
-  #  min_lifetime: 1d
-  #  max_lifetime: 1y
-
-  # Retention policy limits. If set, a user won't be able to send a
-  # 'm.room.retention' event which features a 'min_lifetime' or a 'max_lifetime'
-  # that's not within this range. This is especially useful in closed federations,
-  # in which server admins can make sure every federating server applies the same
-  # rules.
-  #
-  #allowed_lifetime_min: 1d
-  #allowed_lifetime_max: 1y
-
-  # Server admins can define the settings of the background jobs purging the
-  # events which lifetime has expired under the 'purge_jobs' section.
-  #
-  # If no configuration is provided, a single job will be set up to delete expired
-  # events in every room daily.
-  #
-  # Each job's configuration defines which range of message lifetimes the job
-  # takes care of. For example, if 'shortest_max_lifetime' is '2d' and
-  # 'longest_max_lifetime' is '3d', the job will handle purging expired events in
-  # rooms whose state defines a 'max_lifetime' that's both higher than 2 days, and
-  # lower than or equal to 3 days. Both the minimum and the maximum value of a
-  # range are optional, e.g. a job with no 'shortest_max_lifetime' and a
-  # 'longest_max_lifetime' of '3d' will handle every room with a retention policy
-  # which 'max_lifetime' is lower than or equal to three days.
-  #
-  # The rationale for this per-job configuration is that some rooms might have a
-  # retention policy with a low 'max_lifetime', where history needs to be purged
-  # of outdated messages on a very frequent basis (e.g. every 5min), but not want
-  # that purge to be performed by a job that's iterating over every room it knows,
-  # which would be quite heavy on the server.
-  #
-  #purge_jobs:
-  #  - shortest_max_lifetime: 1d
-  #    longest_max_lifetime: 3d
-  #    interval: 5m:
-  #  - shortest_max_lifetime: 3d
-  #    longest_max_lifetime: 1y
-  #    interval: 24h
+#allowed_avatar_mimetypes: ["image/png", "image/jpeg", "image/gif"]
 
 # How long to keep redacted events in unredacted form in the database. After
 # this period redacted events get replaced with their redacted form in the DB.
@@ -1040,30 +986,6 @@ media_store_path: "DATADIR/media_store"
 #
 #max_upload_size: 50M
 
-# The largest allowed size for a user avatar. If not defined, no
-# restriction will be imposed.
-#
-# Note that this only applies when an avatar is changed globally.
-# Per-room avatar changes are not affected. See allow_per_room_profiles
-# for disabling that functionality.
-#
-# Note that user avatar changes will not work if this is set without
-# using Synapse's local media repo.
-#
-#max_avatar_size: 10M
-
-# Allow mimetypes for a user avatar. If not defined, no restriction will
-# be imposed.
-#
-# Note that this only applies when an avatar is changed globally.
-# Per-room avatar changes are not affected. See allow_per_room_profiles
-# for disabling that functionality.
-#
-# Note that user avatar changes will not work if this is set without
-# using Synapse's local media repo.
-#
-#allowed_avatar_mimetypes: ["image/png", "image/jpeg", "image/gif"]
-
 # Maximum number of pixels that will be thumbnailed
 #
 #max_image_pixels: 32M
diff --git a/synapse/config/repository.py b/synapse/config/repository.py
index 0669eaefe..69906a98d 100644
--- a/synapse/config/repository.py
+++ b/synapse/config/repository.py
@@ -112,12 +112,6 @@ def read_config(self, config, **kwargs):
         self.max_image_pixels = self.parse_size(config.get("max_image_pixels", "32M"))
         self.max_spider_size = self.parse_size(config.get("max_spider_size", "10M"))
 
-        self.max_avatar_size = config.get("max_avatar_size")
-        if self.max_avatar_size:
-            self.max_avatar_size = self.parse_size(self.max_avatar_size)
-
-        self.allowed_avatar_mimetypes = config.get("allowed_avatar_mimetypes", [])
-
         self.media_store_path = self.ensure_directory(
             config.get("media_store_path", "media_store")
         )
@@ -272,30 +266,6 @@ def generate_config_section(self, data_dir_path, **kwargs):
         #
         #max_upload_size: 50M
 
-        # The largest allowed size for a user avatar. If not defined, no
-        # restriction will be imposed.
-        #
-        # Note that this only applies when an avatar is changed globally.
-        # Per-room avatar changes are not affected. See allow_per_room_profiles
-        # for disabling that functionality.
-        #
-        # Note that user avatar changes will not work if this is set without
-        # using Synapse's local media repo.
-        #
-        #max_avatar_size: 10M
-
-        # Allow mimetypes for a user avatar. If not defined, no restriction will
-        # be imposed.
-        #
-        # Note that this only applies when an avatar is changed globally.
-        # Per-room avatar changes are not affected. See allow_per_room_profiles
-        # for disabling that functionality.
-        #
-        # Note that user avatar changes will not work if this is set without
-        # using Synapse's local media repo.
-        #
-        #allowed_avatar_mimetypes: ["image/png", "image/jpeg", "image/gif"]
-
         # Maximum number of pixels that will be thumbnailed
         #
         #max_image_pixels: 32M
diff --git a/synapse/config/server.py b/synapse/config/server.py
index 4381a830a..786c18df4 100644
--- a/synapse/config/server.py
+++ b/synapse/config/server.py
@@ -488,6 +488,19 @@ def read_config(self, config, **kwargs):
         # events with profile information that differ from the target's global profile.
         self.allow_per_room_profiles = config.get("allow_per_room_profiles", True)
 
+        # The maximum size an avatar can have, in bytes.
+        self.max_avatar_size = config.get("max_avatar_size")
+        if self.max_avatar_size is not None:
+            self.max_avatar_size = self.parse_size(self.max_avatar_size)
+
+        # The MIME types allowed for an avatar.
+        self.allowed_avatar_mimetypes = config.get("allowed_avatar_mimetypes")
+        if self.allowed_avatar_mimetypes and not isinstance(
+            self.allowed_avatar_mimetypes,
+            list,
+        ):
+            raise ConfigError("allowed_avatar_mimetypes must be a list")
+
         # Whether to show the users on this homeserver in the user directory. Defaults to
         # True.
         self.show_users_in_user_directory = config.get(
@@ -1172,73 +1185,19 @@ def generate_config_section(
         #
         #allow_per_room_profiles: false
 
-        # Whether to show the users on this homeserver in the user directory. Defaults to
-        # 'true'.
+        # The largest allowed file size for a user avatar. Defaults to no restriction.
         #
-        #show_users_in_user_directory: false
-
-        # Message retention policy at the server level.
+        # Note that user avatar changes will not work if this is set without
+        # using Synapse's media repository.
         #
-        # Room admins and mods can define a retention period for their rooms using the
-        # 'm.room.retention' state event, and server admins can cap this period by setting
-        # the 'allowed_lifetime_min' and 'allowed_lifetime_max' config options.
+        #max_avatar_size: 10M
+
+        # The MIME types allowed for user avatars. Defaults to no restriction.
         #
-        # If this feature is enabled, Synapse will regularly look for and purge events
-        # which are older than the room's maximum retention period. Synapse will also
-        # filter events received over federation so that events that should have been
-        # purged are ignored and not stored again.
+        # Note that user avatar changes will not work if this is set without
+        # using Synapse's media repository.
         #
-        retention:
-          # The message retention policies feature is disabled by default. Uncomment the
-          # following line to enable it.
-          #
-          #enabled: true
-
-          # Default retention policy. If set, Synapse will apply it to rooms that lack the
-          # 'm.room.retention' state event. Currently, the value of 'min_lifetime' doesn't
-          # matter much because Synapse doesn't take it into account yet.
-          #
-          #default_policy:
-          #  min_lifetime: 1d
-          #  max_lifetime: 1y
-
-          # Retention policy limits. If set, a user won't be able to send a
-          # 'm.room.retention' event which features a 'min_lifetime' or a 'max_lifetime'
-          # that's not within this range. This is especially useful in closed federations,
-          # in which server admins can make sure every federating server applies the same
-          # rules.
-          #
-          #allowed_lifetime_min: 1d
-          #allowed_lifetime_max: 1y
-
-          # Server admins can define the settings of the background jobs purging the
-          # events which lifetime has expired under the 'purge_jobs' section.
-          #
-          # If no configuration is provided, a single job will be set up to delete expired
-          # events in every room daily.
-          #
-          # Each job's configuration defines which range of message lifetimes the job
-          # takes care of. For example, if 'shortest_max_lifetime' is '2d' and
-          # 'longest_max_lifetime' is '3d', the job will handle purging expired events in
-          # rooms whose state defines a 'max_lifetime' that's both higher than 2 days, and
-          # lower than or equal to 3 days. Both the minimum and the maximum value of a
-          # range are optional, e.g. a job with no 'shortest_max_lifetime' and a
-          # 'longest_max_lifetime' of '3d' will handle every room with a retention policy
-          # which 'max_lifetime' is lower than or equal to three days.
-          #
-          # The rationale for this per-job configuration is that some rooms might have a
-          # retention policy with a low 'max_lifetime', where history needs to be purged
-          # of outdated messages on a very frequent basis (e.g. every 5min), but not want
-          # that purge to be performed by a job that's iterating over every room it knows,
-          # which would be quite heavy on the server.
-          #
-          #purge_jobs:
-          #  - shortest_max_lifetime: 1d
-          #    longest_max_lifetime: 3d
-          #    interval: 5m:
-          #  - shortest_max_lifetime: 3d
-          #    longest_max_lifetime: 1y
-          #    interval: 24h
+        #allowed_avatar_mimetypes: ["image/png", "image/jpeg", "image/gif"]
 
         # How long to keep redacted events in unredacted form in the database. After
         # this period redacted events get replaced with their redacted form in the DB.
diff --git a/synapse/handlers/profile.py b/synapse/handlers/profile.py
index 82950e433..f5f945de0 100644
--- a/synapse/handlers/profile.py
+++ b/synapse/handlers/profile.py
@@ -41,6 +41,8 @@
     create_requester,
     get_domain_from_id,
 )
+from synapse.util.caches.descriptors import cached
+from synapse.util.stringutils import parse_and_validate_mxc_uri
 
 if TYPE_CHECKING:
     from synapse.server import HomeServer
@@ -77,13 +79,15 @@ def __init__(self, hs: "HomeServer"):
         self.request_ratelimiter = hs.get_request_ratelimiter()
 
         self.http_client = hs.get_simple_http_client()
-
-        self.max_avatar_size = hs.config.media.max_avatar_size
-        self.allowed_avatar_mimetypes = hs.config.media.allowed_avatar_mimetypes
         self.replicate_user_profiles_to = (
             hs.config.registration.replicate_user_profiles_to
         )
 
+        self.max_avatar_size = hs.config.server.max_avatar_size
+        self.allowed_avatar_mimetypes = hs.config.server.allowed_avatar_mimetypes
+
+        self.server_name = hs.config.server.server_name
+
         if hs.config.worker.run_background_tasks:
             self.clock.looping_call(
                 self._update_remote_profile_cache, self.PROFILE_UPDATE_MS
@@ -441,44 +445,13 @@ async def set_avatar_url(
                 400, "Avatar URL is too long (max %i)" % (MAX_AVATAR_URL_LEN,)
             )
 
+        if not await self.check_avatar_size_and_mime_type(new_avatar_url):
+            raise SynapseError(403, "This avatar is not allowed", Codes.FORBIDDEN)
+
         avatar_url_to_set: Optional[str] = new_avatar_url
         if new_avatar_url == "":
             avatar_url_to_set = None
 
-        # Enforce a max avatar size if one is defined
-        if avatar_url_to_set and (
-            self.max_avatar_size or self.allowed_avatar_mimetypes
-        ):
-            media_id = self._validate_and_parse_media_id_from_avatar_url(
-                avatar_url_to_set
-            )
-
-            # Check that this media exists locally
-            media_info = await self.store.get_local_media(media_id)
-            if not media_info:
-                raise SynapseError(
-                    400, "Unknown media id supplied", errcode=Codes.NOT_FOUND
-                )
-
-            # Ensure avatar does not exceed max allowed avatar size
-            media_size = media_info["media_length"]
-            if self.max_avatar_size and media_size > self.max_avatar_size:
-                raise SynapseError(
-                    400,
-                    "Avatars must be less than %s bytes in size"
-                    % (self.max_avatar_size,),
-                    errcode=Codes.TOO_LARGE,
-                )
-
-            # Ensure the avatar's file type is allowed
-            if (
-                self.allowed_avatar_mimetypes
-                and media_info["media_type"] not in self.allowed_avatar_mimetypes
-            ):
-                raise SynapseError(
-                    400, "Avatar file type '%s' not allowed" % media_info["media_type"]
-                )
-
         # Same like set_displayname
         if by_admin:
             requester = create_requester(
@@ -503,22 +476,65 @@ async def set_avatar_url(
 
         await self._update_join_states(requester, target_user)
 
-        # start a profile replication push
-        run_in_background(self._replicate_profiles)
-
-    def _validate_and_parse_media_id_from_avatar_url(self, mxc: str) -> str:
-        """Validate and parse a provided avatar url and return the local media id
+    @cached()
+    async def check_avatar_size_and_mime_type(self, mxc: str) -> bool:
+        """Check that the size and content type of the avatar at the given MXC URI are
+        within the configured limits.
 
         Args:
-            mxc: A mxc URL
+            mxc: The MXC URI at which the avatar can be found.
 
         Returns:
-            The ID of the media
+             A boolean indicating whether the file can be allowed to be set as an avatar.
         """
-        avatar_pieces = mxc.split("/")
-        if len(avatar_pieces) != 4 or avatar_pieces[0] != "mxc:":
-            raise SynapseError(400, "Invalid avatar URL '%s' supplied" % mxc)
-        return avatar_pieces[-1]
+        if not self.max_avatar_size and not self.allowed_avatar_mimetypes:
+            return True
+
+        server_name, _, media_id = parse_and_validate_mxc_uri(mxc)
+
+        if server_name == self.server_name:
+            media_info = await self.store.get_local_media(media_id)
+        else:
+            media_info = await self.store.get_cached_remote_media(server_name, media_id)
+
+        if media_info is None:
+            # Both configuration options need to access the file's metadata, and
+            # retrieving remote avatars just for this becomes a bit of a faff, especially
+            # if e.g. the file is too big. It's also generally safe to assume most files
+            # used as avatar are uploaded locally, or if the upload didn't happen as part
+            # of a PUT request on /avatar_url that the file was at least previewed by the
+            # user locally (and therefore downloaded to the remote media cache).
+            logger.warning("Forbidding avatar change to %s: avatar not on server", mxc)
+            return False
+
+        if self.max_avatar_size:
+            # Ensure avatar does not exceed max allowed avatar size
+            if media_info["media_length"] > self.max_avatar_size:
+                logger.warning(
+                    "Forbidding avatar change to %s: %d bytes is above the allowed size "
+                    "limit",
+                    mxc,
+                    media_info["media_length"],
+                )
+                return False
+
+        if self.allowed_avatar_mimetypes:
+            # Ensure the avatar's file type is allowed
+            if (
+                self.allowed_avatar_mimetypes
+                and media_info["media_type"] not in self.allowed_avatar_mimetypes
+            ):
+                logger.warning(
+                    "Forbidding avatar change to %s: mimetype %s not allowed",
+                    mxc,
+                    media_info["media_type"],
+                )
+                return False
+
+        return True
+
+        # start a profile replication push
+        run_in_background(self._replicate_profiles)
 
     async def on_profile_query(self, args: JsonDict) -> JsonDict:
         """Handles federation profile query requests."""
diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index b81180af0..58ef7b958 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -590,6 +590,12 @@ async def update_membership_locked(
                 errcode=Codes.BAD_JSON,
             )
 
+        if "avatar_url" in content:
+            if not await self.profile_handler.check_avatar_size_and_mime_type(
+                content["avatar_url"],
+            ):
+                raise SynapseError(403, "This avatar is not allowed", Codes.FORBIDDEN)
+
         # The event content should *not* include the authorising user as
         # it won't be properly signed. Strip it out since it might come
         # back from a client updating a display name / avatar.
diff --git a/tests/handlers/test_profile.py b/tests/handlers/test_profile.py
index 42ff72a94..d50448b18 100644
--- a/tests/handlers/test_profile.py
+++ b/tests/handlers/test_profile.py
@@ -11,12 +11,13 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-
+from typing import Any, Dict
 from unittest.mock import Mock
 
 import synapse.types
 from synapse.api.errors import AuthError, SynapseError
 from synapse.rest import admin
+from synapse.server import HomeServer
 from synapse.types import UserID
 
 from tests import unittest
@@ -46,7 +47,7 @@ def register_query_handler(query_type, handler):
         )
         return hs
 
-    def prepare(self, reactor, clock, hs):
+    def prepare(self, reactor, clock, hs: HomeServer):
         self.store = hs.get_datastore()
 
         self.frank = UserID.from_string("@1234abcd:test")
@@ -248,3 +249,92 @@ def test_set_my_avatar_if_disabled(self):
             ),
             SynapseError,
         )
+
+    def test_avatar_constraints_no_config(self):
+        """Tests that the method to check an avatar against configured constraints skips
+        all of its check if no constraint is configured.
+        """
+        # The first check that's done by this method is whether the file exists; if we
+        # don't get an error on a non-existing file then it means all of the checks were
+        # successfully skipped.
+        res = self.get_success(
+            self.handler.check_avatar_size_and_mime_type("mxc://test/unknown_file")
+        )
+        self.assertTrue(res)
+
+    @unittest.override_config({"max_avatar_size": 50})
+    def test_avatar_constraints_missing(self):
+        """Tests that an avatar isn't allowed if the file at the given MXC URI couldn't
+        be found.
+        """
+        res = self.get_success(
+            self.handler.check_avatar_size_and_mime_type("mxc://test/unknown_file")
+        )
+        self.assertFalse(res)
+
+    @unittest.override_config({"max_avatar_size": 50})
+    def test_avatar_constraints_file_size(self):
+        """Tests that a file that's above the allowed file size is forbidden but one
+        that's below it is allowed.
+        """
+        self._setup_local_files(
+            {
+                "small": {"size": 40},
+                "big": {"size": 60},
+            }
+        )
+
+        res = self.get_success(
+            self.handler.check_avatar_size_and_mime_type("mxc://test/small")
+        )
+        self.assertTrue(res)
+
+        res = self.get_success(
+            self.handler.check_avatar_size_and_mime_type("mxc://test/big")
+        )
+        self.assertFalse(res)
+
+    @unittest.override_config({"allowed_avatar_mimetypes": ["image/png"]})
+    def test_avatar_constraint_mime_type(self):
+        """Tests that a file with an unauthorised MIME type is forbidden but one with
+        an authorised content type is allowed.
+        """
+        self._setup_local_files(
+            {
+                "good": {"mimetype": "image/png"},
+                "bad": {"mimetype": "application/octet-stream"},
+            }
+        )
+
+        res = self.get_success(
+            self.handler.check_avatar_size_and_mime_type("mxc://test/good")
+        )
+        self.assertTrue(res)
+
+        res = self.get_success(
+            self.handler.check_avatar_size_and_mime_type("mxc://test/bad")
+        )
+        self.assertFalse(res)
+
+    def _setup_local_files(self, names_and_props: Dict[str, Dict[str, Any]]):
+        """Stores metadata about files in the database.
+
+        Args:
+            names_and_props: A dictionary with one entry per file, with the key being the
+                file's name, and the value being a dictionary of properties. Supported
+                properties are "mimetype" (for the file's type) and "size" (for the
+                file's size).
+        """
+        store = self.hs.get_datastore()
+
+        for name, props in names_and_props.items():
+            self.get_success(
+                store.store_local_media(
+                    media_id=name,
+                    media_type=props.get("mimetype", "image/png"),
+                    time_now_ms=self.clock.time_msec(),
+                    upload_name=None,
+                    media_length=props.get("size", 50),
+                    user_id=UserID.from_string("@rin:test"),
+                )
+            )
diff --git a/tests/rest/client/test_profile.py b/tests/rest/client/test_profile.py
index 2860579c2..ead883ded 100644
--- a/tests/rest/client/test_profile.py
+++ b/tests/rest/client/test_profile.py
@@ -13,8 +13,12 @@
 # limitations under the License.
 
 """Tests REST events for /profile paths."""
+from typing import Any, Dict
+
+from synapse.api.errors import Codes
 from synapse.rest import admin
 from synapse.rest.client import login, profile, room
+from synapse.types import UserID
 
 from tests import unittest
 
@@ -25,6 +29,7 @@ class ProfileTestCase(unittest.HomeserverTestCase):
         admin.register_servlets_for_client_rest_resource,
         login.register_servlets,
         profile.register_servlets,
+        room.register_servlets,
     ]
 
     def make_homeserver(self, reactor, clock):
@@ -150,6 +155,157 @@ def _get_avatar_url(self, name=None):
         self.assertEqual(channel.code, 200, channel.result)
         return channel.json_body.get("avatar_url")
 
+    @unittest.override_config({"max_avatar_size": 50})
+    def test_avatar_size_limit_global(self):
+        """Tests that the maximum size limit for avatars is enforced when updating a
+        global profile.
+        """
+        self._setup_local_files(
+            {
+                "small": {"size": 40},
+                "big": {"size": 60},
+            }
+        )
+
+        channel = self.make_request(
+            "PUT",
+            f"/profile/{self.owner}/avatar_url",
+            content={"avatar_url": "mxc://test/big"},
+            access_token=self.owner_tok,
+        )
+        self.assertEqual(channel.code, 403, channel.result)
+        self.assertEqual(
+            channel.json_body["errcode"], Codes.FORBIDDEN, channel.json_body
+        )
+
+        channel = self.make_request(
+            "PUT",
+            f"/profile/{self.owner}/avatar_url",
+            content={"avatar_url": "mxc://test/small"},
+            access_token=self.owner_tok,
+        )
+        self.assertEqual(channel.code, 200, channel.result)
+
+    @unittest.override_config({"max_avatar_size": 50})
+    def test_avatar_size_limit_per_room(self):
+        """Tests that the maximum size limit for avatars is enforced when updating a
+        per-room profile.
+        """
+        self._setup_local_files(
+            {
+                "small": {"size": 40},
+                "big": {"size": 60},
+            }
+        )
+
+        room_id = self.helper.create_room_as(tok=self.owner_tok)
+
+        channel = self.make_request(
+            "PUT",
+            f"/rooms/{room_id}/state/m.room.member/{self.owner}",
+            content={"membership": "join", "avatar_url": "mxc://test/big"},
+            access_token=self.owner_tok,
+        )
+        self.assertEqual(channel.code, 403, channel.result)
+        self.assertEqual(
+            channel.json_body["errcode"], Codes.FORBIDDEN, channel.json_body
+        )
+
+        channel = self.make_request(
+            "PUT",
+            f"/rooms/{room_id}/state/m.room.member/{self.owner}",
+            content={"membership": "join", "avatar_url": "mxc://test/small"},
+            access_token=self.owner_tok,
+        )
+        self.assertEqual(channel.code, 200, channel.result)
+
+    @unittest.override_config({"allowed_avatar_mimetypes": ["image/png"]})
+    def test_avatar_allowed_mime_type_global(self):
+        """Tests that the MIME type whitelist for avatars is enforced when updating a
+        global profile.
+        """
+        self._setup_local_files(
+            {
+                "good": {"mimetype": "image/png"},
+                "bad": {"mimetype": "application/octet-stream"},
+            }
+        )
+
+        channel = self.make_request(
+            "PUT",
+            f"/profile/{self.owner}/avatar_url",
+            content={"avatar_url": "mxc://test/bad"},
+            access_token=self.owner_tok,
+        )
+        self.assertEqual(channel.code, 403, channel.result)
+        self.assertEqual(
+            channel.json_body["errcode"], Codes.FORBIDDEN, channel.json_body
+        )
+
+        channel = self.make_request(
+            "PUT",
+            f"/profile/{self.owner}/avatar_url",
+            content={"avatar_url": "mxc://test/good"},
+            access_token=self.owner_tok,
+        )
+        self.assertEqual(channel.code, 200, channel.result)
+
+    @unittest.override_config({"allowed_avatar_mimetypes": ["image/png"]})
+    def test_avatar_allowed_mime_type_per_room(self):
+        """Tests that the MIME type whitelist for avatars is enforced when updating a
+        per-room profile.
+        """
+        self._setup_local_files(
+            {
+                "good": {"mimetype": "image/png"},
+                "bad": {"mimetype": "application/octet-stream"},
+            }
+        )
+
+        room_id = self.helper.create_room_as(tok=self.owner_tok)
+
+        channel = self.make_request(
+            "PUT",
+            f"/rooms/{room_id}/state/m.room.member/{self.owner}",
+            content={"membership": "join", "avatar_url": "mxc://test/bad"},
+            access_token=self.owner_tok,
+        )
+        self.assertEqual(channel.code, 403, channel.result)
+        self.assertEqual(
+            channel.json_body["errcode"], Codes.FORBIDDEN, channel.json_body
+        )
+
+        channel = self.make_request(
+            "PUT",
+            f"/rooms/{room_id}/state/m.room.member/{self.owner}",
+            content={"membership": "join", "avatar_url": "mxc://test/good"},
+            access_token=self.owner_tok,
+        )
+        self.assertEqual(channel.code, 200, channel.result)
+
+    def _setup_local_files(self, names_and_props: Dict[str, Dict[str, Any]]):
+        """Stores metadata about files in the database.
+
+        Args:
+            names_and_props: A dictionary with one entry per file, with the key being the
+                file's name, and the value being a dictionary of properties. Supported
+                properties are "mimetype" (for the file's type) and "size" (for the
+                file's size).
+        """
+        store = self.hs.get_datastore()
+
+        for name, props in names_and_props.items():
+            self.get_success(
+                store.store_local_media(
+                    media_id=name,
+                    media_type=props.get("mimetype", "image/png"),
+                    time_now_ms=self.clock.time_msec(),
+                    upload_name=None,
+                    media_length=props.get("size", 50),
+                    user_id=UserID.from_string("@rin:test"),
+                )
+            )
+
 
 class ProfilesRestrictedTestCase(unittest.HomeserverTestCase):