I will note that on unix the user is also free to update the machine's mime types registry (that's more than half the point of the mimetypes module).  Usually this is only done by installed software...as I believe is the case on Windows as well.

That said, there should be a way to explicitly bypass this loading of local data for a program that wishes to use only the Python supplied types.  And indeed, this is possible: just pass an empty list of filenames to init.  This bypasses the windows registry lookup.  (Note that this could be better documented...it is not made explicit that an empty list is different from not specifying a list or specifying it as None, but it is).