Return a new ``<Default>`` element with attributes set to parameter
        values.

def new(ext, content_type):
        """
        Return a new ``<Default>`` element with attributes set to parameter
        values.
        """
        xml = '<Default xmlns="%s"/>' % nsmap['ct']
        default = parse_xml(xml)
        default.set('Extension', ext)
        default.set('ContentType', content_type)
        return default