docs: def _from_stream(cls, stream, blob, filename=None):
          """
          Return an instance of the |Image| subclass corresponding to the
          format of the image in *stream*.
          """
          image_header = _ImageHeaderFactory(stream)
          if filename is None:
              filename = 'image.%s' % image_header.default_ext
          return cls(blob, filename, image_header)