I was having some trouble with that too on the original GMTK page (part of why I was late, but I got submitted after). What happens  is you create a page for your game, and you can upload multiple times throughout the page for the game. You don't submit a file as much as you submit a page with a file on it. What the upload button does is say "hey you can press this button and create a page." You can also just submit a page you already have (you probably want to create it during the jam). Hope this helps!