def is_markdown(filename: str) -> bool:
    """Checks whether a given file has a known markdown extension.

    Args:
        filename: name of file to check

    Returns:
        bool
    """
    markdown_extensions = ['md', 'mkd', 'markdown', 'mdown', 'mkdn', 'txt']

    if filename.split('.')[-1] in markdown_extensions:
        return True
    else:
        return False