[@Light the Dark]

This is interesting. I grabbed a true gif file with a GIF89a file header, and [i]that[/i] wasn't accepted either. While I'd love to blame Google and their trash .webp format here, this seems to be an issue with any animated file.

[b]Interestingly[/b] though, animated PNGs work just fine. See https://www.roleplayerguild.com/users/legendbegins/images/741a1436-24b6-4de8-bceb-84adb7420333.

The above could be a workaround for your case [@BrokenPromise]. You can do the conversion at https://ezgif.com/apng-maker.