Add support for gif attachment

[StCyr]

Support for images has been added with https://github.com/nextcloud/social/pull/626

We should add support for gif

[v0idkr4ft]

Just need to edit mime types some where to add MIME Image/Gif, if someone could point to where that is please. I might dig around later.

[v0idkr4ft]

Ok did some digging around. GIF mime type is enabled, but mastodon turns gifs into mp4's it appears after inspecting them from mastodon.social so the MIME type would be video/mp4 if Social is receiving them as mp4 and not gifs we'd need a different function as the save to cache and resize function that's being done may only work for image and not video. at least it didn't work when i tried. it also might just not work for gifs and will need to define another exception/function for them. but may poke around some more when i has the time. file where mime types are set is in apps/social/lib/Service/CacheDocumentService.php

196	public function filterMimeTypes(string $mime) {
197
198	$allowedMimeType = [
199	'image/jpeg',
200	'image/gif',
201	'image/png'