Tip for MacOS users:
If you are a plugin developer, or if the plugin zip file is expanded when downloading, I have found the zip file should be (re)created using the Terminal app rather than the Finder/Compress utility. In my experience, the latter always produces the 'not a valid plugin' error message.
I use the following Terminal command ...
zip -r [foldername].zip [foldername]/
Hope this helps.
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