This change mirrors a similar fix that we recently did for Review Board.

This fixes two issues with RBTools' MIME-type detection:


Some versions of Python break if you try to close an already-closed

   pipe, and this had happened on one customer's machine. file may

   close the input stream once it's read enough data to make a

   determination, and our attempt to close would then fail.


Errors executing or communicating with file would crash.

Ran unit tests.