Maybe your #mastodon instance is playing tricks while uploading GIF files (dunno why it's not happening with other image formats):
413 Entity too large.
In our case this is a dockerized instance with `jwilder/nginx-proxy` in the middle, the fix is easy:
1. Use a nginx config volume: `./data/conf:/etc/nginx/conf.d`
2. create a `client_max_body_size.conf`file in the volume with "client_max_body_size 100m;" inside.
3. Restart the instance