diff --git a/caput/memh5.py b/caput/memh5.py
index cedaa677..72c5ad0b 100644
--- a/caput/memh5.py
+++ b/caput/memh5.py
@@ -479,12 +479,13 @@ def from_file(
         selections=None,
         convert_dataset_strings=False,
         convert_attribute_strings=True,
-        file_format=fileformats.HDF5,
+        file_format=None,
         **kwargs,
     ):
         """Create a new instance by copying from a file group.
 
-        Any keyword arguments are passed on to the constructor for `h5py.File` or `zarr.File`.
+        Any keyword arguments are passed on to the constructor for `h5py.File` or
+        `zarr.File`.
 
         Parameters
         ----------
@@ -505,8 +506,8 @@ def from_file(
             Try and convert attribute string types to unicode. Default is `True`.
         convert_dataset_strings : bool, optional
             Try and convert dataset string types to unicode. Default is `False`.
-        file_format : `fileformats.FileFormat`
-            File format to use. Default `fileformats.HDF5`.
+        file_format : `fileformats.FileFormat`, optional
+            File format to use. Default is `None`, i.e. guess from the name.
 
         Returns
         -------
@@ -517,6 +518,9 @@ def from_file(
         if comm is None:
             comm = mpiutil.world
 
+        if file_format is None:
+            file_format = fileformats.guess_file_format(filename)
+
         if comm is None:
             if distributed:
                 warnings.warn(
@@ -595,7 +599,7 @@ def to_file(
         hints=True,
         convert_attribute_strings=True,
         convert_dataset_strings=False,
-        file_format=fileformats.HDF5,
+        file_format=None,
         **kwargs,
     ):
         """Replicate object on disk in an hdf5 or zarr file.
@@ -614,9 +618,12 @@ def to_file(
             understands. Default is `True`.
         convert_dataset_strings : bool, optional
             Try and convert dataset string types to bytestrings. Default is `False`.
-        file_format : `fileformats.FileFormat`
-            File format to use. Default `fileformats.HDF5`.
+        file_format : `fileformats.FileFormat`, optional
+            File format to use. Default is `None`, i.e. guess from the name.
         """
+        if file_format is None:
+            file_format = fileformats.guess_file_format(filename)
+
         if not self.distributed:
             with file_format.open(filename, mode, **kwargs) as f:
                 deep_group_copy(
@@ -627,9 +634,6 @@ def to_file(
                     file_format=file_format,
                 )
         else:
-            if file_format is None:
-                file_format = fileformats.guess_file_format(filename)
-
             _distributed_group_to_file(
                 self,
                 filename,
@@ -1665,7 +1669,7 @@ def from_file(
         detect_subclass=True,
         convert_attribute_strings=None,
         convert_dataset_strings=None,
-        file_format=fileformats.HDF5,
+        file_format=None,
         **kwargs,
     ):
         """Create data object from analysis hdf5 file, store in memory or on disk.
@@ -1704,11 +1708,14 @@ def from_file(
             Axis selections can be given to only read a subset of the containers. A
             slice can be given, or a list of specific array indices for that axis.
         file_format : `fileformats.FileFormat`
-            File format to use. Default `fileformats.HDF5`.
+            File format to use. Default is `None`, i.e. guess from file name.
         **kwargs : any other arguments
             Any additional keyword arguments are passed to :class:`h5py.File`'s
             constructor if *file_* is a filename and silently ignored otherwise.
         """
+        if file_format is None and not is_group(file_):
+            file_format = fileformats.guess_file_format(file_)
+
         if file_format == fileformats.Zarr and not zarr_available:
             raise RuntimeError("Unable to read zarr file, please install zarr.")
 
diff --git a/caput/tests/test_memh5.py b/caput/tests/test_memh5.py
index 9a9be540..bcdaa46c 100644
--- a/caput/tests/test_memh5.py
+++ b/caput/tests/test_memh5.py
@@ -164,13 +164,11 @@ def test_to_from_file(test_file, file_open_function, file_format):
     with file_open_function(test_file, "r") as f:
         assertGroupsEqual(f, m)
 
-    m.to_file(
-        test_file + ".new",
-        file_format=file_format,
-    )
+    new_name = f"new.{test_file}"
+    m.to_file(new_name, file_format=file_format)
 
     # Check that written file has same structure
-    with file_open_function(test_file + ".new", "r") as f:
+    with file_open_function(new_name, "r") as f:
         assertGroupsEqual(f, m)