I got it working right now with 0 friction, see the README.md file on my game page (you do have to scroll to the bottom).
The edit UI says:
Any additional files you upload will be made available for download.
That seems to be true in my experience. All I had to do was attach a second file (after the first HTML file that’s played online) and tell it the file type… It doesn’t seem to surface the download on the submission page (which may be what you were fighting with) - but as soon as someone clicks thru to the actual game page, there it is at the bottom…
That is kind of annoying though. 😕
Anyways, maybe this helps!