The MIME types main- and sub-label can both start with <tt>x-</tt>,
        which indicates that it is a non-registered name. Of course, after
        registration this flag can disappear, adds to the confusing
        proliferation of MIME types. The simplified string has the
        <tt>x-</tt> removed and are translated to lowercase.

def simplify(cls, content_type):
        """
        The MIME types main- and sub-label can both start with <tt>x-</tt>,
        which indicates that it is a non-registered name. Of course, after
        registration this flag can disappear, adds to the confusing
        proliferation of MIME types. The simplified string has the
        <tt>x-</tt> removed and are translated to lowercase.
        """
        matchdata = MEDIA_TYPE_RE.match(content_type)
        if matchdata is None:
            return None
        wrap = lambda s: re.sub(UNREG_RE, '', s.lower())
        (media_type, subtype) = matchdata.groups()
        return '%s/%s' % (wrap(media_type), wrap(subtype))