think its file size related @Marc_Carlyon , @Josa_Booba
the file size limitation for images is 4096kb.
GIFs should be ok theyre not blocked, testing one saved from Giphy
Saved to desktop then uploaded using the button on posting,
File Size 313k (scaled to 50% in post preview window after upload)
Testing uploaded GIF made using ScreenToGif , filesize 5Mb
-should have appeared here-
[EDIT] : so it looks like rather than warning about a filesize being too large, it could just ignore it.
Second Gif, shortened and 340kb
i would say its down to the file size, but ill have a dig around to see if theres a way of inserting a warning about file sizes. for now, i would say if it doesnt appear in the post preview then its not going to make it to live post
if you would like, pop me over the gif via googledrive or something and ill test further.