My terrible confession: I just never got around to setting up thumbnail generation for gifs! I only set it up for pngs and jpgs :(
At some point, I'm probably gonna do a big sweep through the thumbnail generation and try to set it up for a bunch of additional file formats (including gifs). For the time being though, sorry!
(Out of curiosity, do you picture gifs having animated thumbnails in impart? Or would you rather it just be the first frame? I'll probably implement both (assuming it isn't a nightmare to figure out) and have an option for it but idk which'd be a preferable default lol)