Return True if the media type is a valid form media type.

def is_form_media_type(media_type):
    """
    Return True if the media type is a valid form media type.
    """
    base_media_type, params = parse_header(media_type.encode(HTTP_HEADER_ENCODING))
    return (base_media_type == 'application/x-www-form-urlencoded' or
            base_media_type == 'multipart/form-data')