Ah, and, of course, TXT, PROPERTIES and XML; not sure if those are automatically included.
Also, as per what you can do in the current Developer forum, of course you should be able to upload sounds and listen to them, directly (which you can currently do in the Sourceforge Forum), meaning WAV and MP3 (WAV are not supported anymore in the engine, but it is for the lossless source, as well as 22.214.171.124 and previous compatibility).
Another reason, I forgot to mention, why GIF is a must is that custom cursors are GIF only (afaik), in case anyone wants to share a custom cursor on the forum. Sadly, no developers have added PNG support for custom cursors; so, I think that is the only remaining item bogged down to GIF only support (thus only visibly pixilated cursors are possible, meh). Other things in the engine accept GIF, but I don't see much point, there, since you can rather use PNG.
Going back to the APNG or MNG diatribe, actually, APNG is an extension of PNG; so I guess that supporting PNG will allow to upload APNG, as well (not 100% sure). Instead, MNG is its own format. I've no clue what is prevalent, but, as a GIMP user and as per 2.8, MNG is natively supported, while APNG requires the addition of a custom plug-in, to be separately downloaded.
Basically, MNG is better, and more like it, but it is problematic to support, or anyways got close to zero support from browsers, after the first release, thus APNG was developed, that should be more easily supportable, as long as you already support PNG, but I believe both have scarce support, at the end, afaik (this is why the crappy GIF is still so popular for animated images on the web).
Side note, another way to have advanced animation would be ANI (that is related to WAV), that is normally used only for Windows animated cursors, afaik, tho (not suggesting to support this).