Support AVIF in inline-preview

[Dominik-K]

It would be nice if Zulip starts to support using AVIF-images in inline-previews (see #9081) which are getting traction on (some) websites. AVIF is designed to supersede WebP, JPEG, PNG and GIF and is supported by all major browsers nowadays (source: https://caniuse.com/?search=AVIF ).

[timabbott]

The caniuse chart is looking pretty good finally; started https://chat.zulip.org/#narrow/stream/3-backend/topic/Support.20AVIF.20in.20inline-preview.20.2329985/near/1795270 to discuss changing this.

[zulipbot]

Hello @zulip/server-misc members, this issue was labeled with the "area: uploads" label, so you may want to check it out!