You could always set up a webserver on your mac and make sure it reports the mime type for .xpi properly.
I have a feeling your mac is being too smart for its own good and looking at the alternate streams on the file. Either that, or you just renamed the file without changing the extension; i.e., file.zip -> file.xpi.zip. Try doing the rename in the terminal as opposed to the finder. Maybe the Get Info command will let you change the file type...
I hope all of this helps
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