Usually when that happens you just have to change the submission file and upload the gif file again. Usually works.

I'm no artist nor uploader of... anything really! But if I were to upload animated stuff on to this site I'd just upload a static image to FA itself and then have a link (maybe to e621) for the actually animated bit.

If it is a .gif it should just upload like a normal file, but I don't believe we can upload literal videos to FA. So as for posting an animated video then that would be something where you post a .gif and link to an external webpage where you can post videos.