Fix some weird attachment issues if they don't exist locally

If the attachment type is "unknown", try to guess the actual attachment type from the filename. If the server's media proxy fails to cache the file, try to fall back to the remote URL.

An example of a troublesome post for me is https://mastodon.world/@sugacofi/111155199996838941. My server didn't have the attachment cached, it had type of "unknown" (even though the post on their server is correctly marked as "image").

Should be backported to 23.08.