Previously logging in reviewboard/reviewboard/attachments was 

un-named logging. This means it's hard to determine where the 

error came from in the logs.
To fix this a global logger variable was defined in each file 

that adds the location of the file to the error message.

Added unit test for unregister_mimetype_handler that passes it an 

unregistered handler. This should cause an error to be logged. The 

unit test listens to whether or not the logger ran.
Ran ./tests/runtests.py reviewboard.attachments and passed all 32 tests.