docs: def renderHTTP(self, ctx):
          """
          Extract the data from the I{uploaddata} field of the request and pass
          it to our callback.
          """
          req = inevow.IRequest(ctx)
          if req.method == 'POST':
              udata = req.fields['uploaddata']
              self.cbGotMugshot(udata.type.decode('ascii'), udata.file)
          return rend.Page.renderHTTP(self, ctx)