:param category: audio|image|office|text|video
        :param extension: file extension

def file_name(self, category=None, extension=None):
        """
        :param category: audio|image|office|text|video
        :param extension: file extension
        """
        extension = extension if extension else self.file_extension(category)
        filename = self.generator.word()
        return '{0}.{1}'.format(filename, extension)