I've tried to upload the .avi file of some fan animations or even link to the youtube and all I get is an error message.

Right now, files have to be either flash (.swf) or VP8/VP9 codec using video (.webm). That means other formats needs to be converted. With youtube, videos are already in webm format, you just need to rip that one instead of h264 (.mp4) version which many ripping programs and -sites default to.