Mastodon: media upload feature does not make it clear what's allowed

[wchristian]

The errors you get from uploading a media file sadly come from the server, after upload, and are quite cryptic.

I think:

1. media uploads should be checked against simple filename checks before uploading
2. error messages should contain info on what's allowed
3. possibly also make a tooltip for the media button (and no, that doesn' obsolete 2, due to non-hoverable hardware)

[dracos]

For 1, 459bbfa4b21c20809a142c41c389b7a5edd28bd8 (May 2017) prevented uploads unless they were an accepted type, which works in many browsers, but not Edge: https://caniuse.com/#search=accept
For 3, have submitted #8443.