Multiple instances of wrong MIME type in file uploads

[syldrathecat]

Steps to reproduce

I don't have a reproduction for this and I'm not sure what causes it, but people who use Element X and upload media frequently result in uploads with invalid mimetypes specified -- in both the event JSON and in the mimetype passed to the media upload endpoint.

e.g. we see events like this

 "content": {
    <...>
    "info": {
      <...>
      "mimetype": "image/*",
      <...>
    },
    "msgtype": "m.image",
    "url": "<...>"
  },

Similarly, we have over 500 pieces of media in our database with the mime type image/*, which causes matrix-media-repo to fail to generate thumbnails.

---

Also, a separate issue maybe:

I have noticed that images uploaded in to encrypted rooms also get uploaded to the media upload endpoint with the "wrong" (i.e. original) mimetype. Encrypted media should be uploaded with the mimetype application/octet-stream to match the behavior of other clients.

Note: This is probably an information leak as it discloses to home-servers the types of files that users are sharing in E2EE chats.

(It is also inconvenient from the point of view of administration and being able to account encrypted media uploads.)

Outcome

--

Your phone model

No response

Operating system version

No response

Application version and app store

No response

Homeserver

No response

Will you send logs?

No

Are you willing to provide a PR?

No