HarmonyOS pixelMap格式的图片如何转成文件流格式？

参考如下写法：

const dateStr = (new Date().getTime()).toString()
let uri= getContext(this).filesDir + `/${dateStr + '.jpg'}`;
let file: fs.File = fs.openSync(uri, fs .OpenMode.WRITE_ONLY | fs.OpenMode.CREATE);
let imagePacker: image.ImagePacker = image.createImagePacker();
let packOpts: image.PackingOption = { format: 'image/jpeg', quality: 100 };
await imagePacker.packToFile(pixelMap , file.fd, packOpts);
fs.closeSync(file);