The downloaded plugin is not a zip archive
You've downloaded a copy of your plugin and it does not end up as a .zip archive file?
This can happen in some browsers (like Safari) that have been set to automatically open any downloaded files. The easiest way to resolve this, is to select all the folder contents and make them an archive again. Make sure to then name the archive the same as the folder:
Another option is to use a different browser (like Chrome) that does not automatically extract the original zip archive.