def check_md_file(md_file_name: str) -> str:
    """
    Check markdown (md) file.

    Checks that the provided file has a .md extension.

    :param md_file_name: A string
    :return: the md file name.
    """
    if md_file_name and ".md" not in md_file_name:
        md_file_name += ".md"
    return md_file_name