If I understand Steve's comments, the mime type is used by Windows, so application/zip lets Windows know that this filetype is fundamentally a zip file (and so it'll offer to open it with your zip program if you right click, stuff like that).

pxzw - the "w" on the end marks it as a GUI script, to be run by pythonw.exe. The x is a "test mode" thing for development builds of Python. You wouldn't see it in a production build (the production filetypes are pyz and pyzw).