def is_filetype(filename: str) -> bool:
    """
    Return true if fname is ends with .csv, .xlsx, or .xls.
    Otherwise return False.

    :filename:      filename string

    Returns bool
    """
    cfname = filename.lower()

    if cfname.endswith(".csv") and not cfname.startswith("pdappend"):
        return True

    elif cfname.endswith(".xlsx"):
        return True

    elif cfname.endswith(".xls"):
        return True

    return False