public function getFormatMimeType($format)
    {
        $format = $this->normalizeFormat($format);
        $result = '';
        switch ($format) {
            case static::FORMAT_PNG:
            case static::FORMAT_JPEG:
            case static::FORMAT_GIF:
            case static::FORMAT_XBM:
                $result = 'image/' . $format;
                break;
            case static::FORMAT_WBMP:
                $result = 'image/vnd.wap.wbmp';
                break;
        }

        return $result;
    }