def ext(self):
        """
        Canonical file extension for this image e.g. ``'png'``. The returned
        extension is all lowercase and is the canonical extension for the
        content type of this image, regardless of what extension may have
        been used in its filename, if any.
        """
        ext_map = {
            'BMP': 'bmp', 'GIF': 'gif', 'JPEG': 'jpg', 'PNG': 'png',
            'TIFF': 'tiff', 'WMF': 'wmf'
        }
        format = self._format
        if format not in ext_map:
            tmpl = "unsupported image format, expected one of: %s, got '%s'"
            raise ValueError(tmpl % (ext_map.keys(), format))
        return ext_map[format]