Return an instance of the |Image| subclass corresponding to the
        format of the image in *stream*.

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)