The getfile() method located around line 106 of backends/blobfiles.py
should open files for both reading and writing, otherwise changes to msg
and file content within extensions and detectors can not be committed
because the .tmp extension can't be removed.

This:

    def getfile(self, classname, nodeid, property):
        filename = self.filename(classname, nodeid, property)
        f = open(filename, 'rb')

is changed to this:

    def getfile(self, classname, nodeid, property):
        filename = self.filename(classname, nodeid, property)
        f = open(filename, 'r+b')

fixing this particularly vexing issue.

The file handle is not retained in the current code so this change no longer 
makes sense. Thanks for the submission anyway.