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

@classmethod
def _from_stream(cls, stream, blob, filename=None):

    image_header = _ImageHeaderFactory(stream)
    if (filename is None):
        filename = ('image.%s' % image_header.default_ext)
    return cls(blob, filename, image_header)
