Lemmy doesn’t support video uploads yet. Even if it did, since instances are hosted privately (people rolling up their own servers or VPSes), I doubt anyone would actually allow video uploads. They just take too much space. Maybe some instances will allow it, but with a size limit, like a 10MB max file size for video files. Still, I doubt there will be many instances like this.

Regarding images and gifs, yes, you can currently upload them on Lemmy. You can upload as many as you want in a post, but some instances have also made filesize limits on image file size. Why? Well, many phones take very HQ pics nowadays and even if compression was set to a very high level, that still generates images of 5MB+ per image… which is a lot. The servers would run out of storage space within weeks. So, A rational file size for images would be somewhere below 1 or 2MB, which is what some instances have enforced.