def detect_tag(filename):
    """Return type and position of ID3v2 tag in filename.
    Returns (tag_class, offset, length), where tag_class
    is either Tag22, Tag23, or Tag24, and (offset, length)
    is the position of the tag in the file.
    """
    with fileutil.opened(filename, "rb") as file:
        file.seek(0)
        header = file.read(10)
        file.seek(0)
        if len(header) < 10:
            raise NoTagError("File too short")
        if header[0:3] != b"ID3":
            raise NoTagError("ID3v2 tag not found")
        if header[3] not in _tag_versions or header[4] != 0:
            raise TagError("Unknown ID3 version: 2.{0}.{1}"
                           .format(*header[3:5]))
        cls = _tag_versions[header[3]]
        offset = 0
        length = Syncsafe.decode(header[6:10]) + 10
        if header[3] == 4 and header[5] & _TAG24_FOOTER:
            length += 10
        return (cls, offset, length)