The itchio site has a guide on how to integrate with the app for full compatibility.
Hmm, looks like windows was having an issue with the Bzip2 compression I used on the original upload. I re-upped the file with default windows compression and it extracts fine now (tested from the downloads section), but the app seems to be caching the old download and failing because it no longer exists.
Not sure how to remedy that myself but already sent off a request to support, in the meantime the game can be downloaded and played manually. Sorry about the issue!
Edit: the game installed properly after restarting my machine , if you're getting an "invalid download" error in the app restarting might help!