def METHOD_NAME(self, m_read_bed_file, extension):
    """load_mask_sites should handle files that end with .bed with any capitalization as a bed"""
    m_read_bed_file.return_value = [3,4]
    assert utils.load_mask_sites("mask.%s" % extension) == [3,4]
    m_read_bed_file.assert_called_with("mask.%s" % extension)