We had a few issues that occurred in some environments when trying to

detect the mimetype of a file from its contents. These were rare,

dependent on Python version behavior and the implementation of file on

the system, but have been verified with one customer in particular:


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.


We didn't set the mimetype variable if file returned a non-0 exit

   code, which resulted in later usage of an undefined variable.


Errors executing or communicating with file would crash, and even

   if caught higher up it'd leave the uploaded file truncated (the file

   pointer wouldn't be at 0).


This change fixes all these through enhanged exception handlers and

checks, an unconditional reset of the uploaded file's file pointer, and

a fallback value for the mimetype.

All unit tests pass.
Notably, I cannot reproduce this bug in my environment, but I can verify

the cause in the affected customer's environment and that this would

address what we know about that crash.