docs: 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))