I don't think you can upload a video file to this site. You have to upload to YT or some other service and then just put a link to your video in your post here.
YT is YouTube. "Upload a file" is typically used for pictures. Video files are usually too big to be uploaded. I post many videos here but the video actually resides on YouTube's servers and is linked in my post. Here's an example:
The link to the file below is actually: https ://youtu.be/eWODGffdLCM