I's somewhat stupid, but the installer tries to guess if it's an archive by the file extension: https://github.com/itchio/hush/blob/master/installer_for_ext.go
If you rename the archive to end in '.tar.gz' I think it will work properly.
(I don't know if you done this, but it's not updated in the app or on the webpage yet.)
If you are not uploading manually, but use butler, than it's a bug in butler, I assume.