Steps to reproduce
1. Try to upload a new plugin to the Modules and plugins database.
2. Get some cryptic error message about the zip file being in the wrong format.
3. (Optional) waste half an hour trying to make a zip file on a mac, before realising that it adds an invisible __MACOS file that the plugins database will reject.
4. So, unzip the code in the file-picker, and re-name the top-level folder to the name of your plugin, like code_checker.
5. Re-zip the code and try to submit the form again.
6. (optional) swear a lot in developer chat.
7. Un-zip the zip again, and find that filepicker has re-named to top-level folder in the zip to 0.
That is crazy. And, of course you could use another form to reproduce this, e.g. forum attachments.