docs: def mimetype_params(self):
          """The mimetype parameters as dict. For example if the
          content type is ``text/html; charset=utf-8`` the params would be
          ``{'charset': 'utf-8'}``.
  
          .. versionadded:: 0.5
          """
  
          def on_update(d):
              self.headers["Content-Type"] = dump_options_header(self.mimetype, d)
  
          d = parse_options_header(self.headers.get("content-type", ""))[1]
          return CallbackDict(d, on_update)