Lemmy, as other similar sites, is meant to function as a link aggregator, not data storage. Therefore, usually any large files such as videos are not allowed to be stored at the service itself. You should truly use any service meant for storing videos and make a post linking that other service with your video, in this case.

But I agree, a documentation describing this topic would be nice. There might be one already, but I am not sure about it at all. And there should be a specific error notification, not a JSON error, I would say. That might be up for a new issue.