@classmethod
def _ap_compute_external_mimetype(cls, package_name, unused_a_type,
    ext_class_name):
    """
        Return the string value of the external mime type for the given
        type in the given package having the given external name (probably
        derived from :meth:`_ap_compute_external_class_name_from_concrete_class`).

        For example, given the arguments ('nti.assessment', FooBar, 'FooBar'), the
        result will be 'application/vnd.nextthought.assessment.foobar'.

        Subclasses may override.
        """
    local = package_name.rsplit('.', 1)[-1]
    return str('application/vnd.nextthought.' + local + '.' +
        ext_class_name.lower())
