Check if the provided file is a valid file for this plugin.

        :arg filename: the path to the file to check.

def valid_file(cls, filename):
        """ Check if the provided file is a valid file for this plugin.

        :arg filename: the path to the file to check.

        """
        return not os.path.isdir(filename) \
            and os.path.basename(filename).startswith('Session ') \
            and filename.endswith('.mqo')