AVIF (animated AVIF) support

Problem

Right now, when you upload an (animated) AVIF file, Fluxor embeds a video player but plays nothing.

Proposed solution

Please add (animated) AVIF support as an alternative to GIFs. Uploading an AVIF should be like uploading a GIF or other image. Avif is supported by all modern browsers now: https://caniuse.com/?search=avif It's also supported by Discord and works on mobile, and it just shows them as if they're regular GIFs.

Notes (optional)

(This is different from the other request for AVIF stickers and emoji. This request is for just uploading and embedding AVIFs in chat in the same way you would share other images or gifs.)