Spec is unclear about what is stored in the content repository

[jimmycuadra]

Confusing/unclear things about the content repository module in the client-server spec:

• The module is described with the terms "content" and "attachments" without ever defining what they mean.
• Content stored in the repository is referred to in URLs and JSON objects as "media" rather than "content" or "attachments." This implies that it is intended for images, audio, and video, but is never actually stated.
• Other than asking for a Content-Type header, the APIs seem to allow storage of arbitrary binary files, which is confusing since the content uploaded is sometimes referred to as "media".
• The thumbnail API seems to assume that the content uploaded was an image, but doesn't mention anything about how the API should behave if the original content was not an image.

[jimmycuadra]

I don't think #2068 totally covered what was reported in this issue. Specifically, it's still never stated explicitly what type of data can be uploaded and stored via this API. The words "content" and "media" are still used as they were when I originally reported this, and the high level description of the module now refers to the content as "files" but still does not state whether this means arbitrary binary data, or whether the data needs to be some format that the server understands. The follow up issue matrix-org/matrix-spec#453 further implies that only "media" (image, video, audio) files are expected, but it's still not clear.