The way videos are handled on reddit is such a mess.
On subreddits that don't allow videos, you can still sometimes post a video shorter than 60 seconds, and reddit claims it will be converted to a GIF, but doesn't actually do that and in fact just posts the video (but with sound stripped if any). Oh, and it only works sometimes.
https://www.reddit.com/r/proceduralgeneration/comments/17y41w7/mods_can_we_allow_videos_in_this_subreddit_please/
If anyone knows more about the details, I'd love to hear about it. It's impossible to google for; no results discuss these details.