Internal server error when uploading encrypted image with correct mimetype

[krille-chan]

Description

If the client tries to upload an encrypted image with the Content-Type for example "image/jpeg", the server response with "Internal server error". For encrypted images I need to change the Content-Type to "application/octet-stream".

Steps to reproduce

• Build a fancy Matrix client with file encryption
• Encrypt an image file
• Upload the image with the Content-Type of the unencrypted image mimetype

Expected:
The server recognized that the file is encrypted and handles it like a blob and does not respond with an internal server error.

Actual:
Can't upload the image without an internal server error.

Version information

• Homeserver: https://famedly.de

If not matrix.org:

• Version: Can tell you this later today

• Install method: Docker, more information will come later today

• Platform: Debian, more information will come later today

[clokep]

@ChristianPauly Is this a client that you're developing?

Taking a look at the spec for encrypted attachments it doesn't seem to specify what the mime type should be in that case.

My guess is that this is breaking when attempting to generate the thumbnail for unknown data, that seems like not great of Synapse. It would be interesting to see the stack trace in the Synapse logs when this happens.

[clokep]

A bit more thoughts:

• Including the actual content type of encrypted is a metadata leak.
• There isn't a recommendation for what should be used for the content type, but a octet stream is reasonable, application/x-riot-encrypted is another idea that came up. It is also possible to not provide a content type.
• The 500 error seems to be a duplicate of Errors from thumbnailing due to bad format give a 500 #3763

I'm going to close this as a duplicate, but please shout if this seems different.