@eveh have you tried to change the settings at
ACP > Settings > Uploads ?
try adding .gif to the list...
alternatively, you can use "tenor gif" plugin...NodeBB, now with GIF support!
... because who doesn't like GIFs? Immature, you say? Waste of time, eh? Totally. NodeBB now has a plugin allowing you to search and reply with GIFs. It is a...favicon
NodeBB Community (community.nodebb.org)
Gif Making GIF