Thanks for reporting as @pkiller mentioned this should be fixed if you create the system folder. I have also fixed it on master as well as the error messages so you should get a better error message if the upload fails.
The "top downloads" pie chart is a little misleading, actually. The package manager itself doesn't have any visibility on package downloads because those are done through npm, and not nbbpm itself. What it does track is how many people ask nbbpm to suggest a version to download.
For example, if you run v1.8.1 and you want to download a plugin, the latest version might not be compatible, so the ACP will query nbbpm for a suggested version. That's counted as a "download" (since the "suggest" call is done behind the scenes and is opaque to the end user).
So you could be seeing true downloads... people downloading your plugins... or someone's just hammering GET /api/v1/suggest?package=nodebb-plugin-prometheus a lot 😄