Scan a filename for it's extension.

    :param filename: string of the filename
    :return: the extension off the end (empty string if it can't find one)

def ext_from_filename(filename):
    """ Scan a filename for it's extension.

    :param filename: string of the filename
    :return: the extension off the end (empty string if it can't find one)
    """
    try:
        base, ext = filename.lower().rsplit(".", 1)
    except ValueError:
        return ''
    ext = ".{0}".format(ext)
    all_exts = [x.extension for x in chain(magic_header_array,
                                           magic_footer_array)]

    if base[-4:].startswith("."):
        # For double extensions like like .tar.gz
        long_ext = base[-4:] + ext
        if long_ext in all_exts:
            return long_ext
    return ext