def set_scheme(illust_id, type):
    """
    Set the type of illust_id.

    file: points to an image, and folder: points to a directory.
    """
    assert type in ('folder', 'file')
    assert ':' in illust_id

    rest = illust_id.split(':', 1)[1]
    return '%s:%s' % (type, rest)