Return the temporary file to use.

def get_fileobject(self, dir=None, **kwargs):
        '''Return the temporary file to use.'''
        if dir is None:
            dir = os.path.normpath(os.path.dirname(self._path))
        descriptor, name = tempfile.mkstemp(dir=dir)
        # io.open() will take either the descriptor or the name, but we need
        # the name later for commit()/replace_atomic() and couldn't find a way
        # to get the filename from the descriptor.
        os.close(descriptor)
        kwargs['mode'] = self._mode
        kwargs['file'] = name
        return io.open(**kwargs)