HarmonyOS 鸿蒙Next usbManager发送文件怎么发？

如何查询指定的文本文件呢，然后返回string

看下下面这个示例呢

//usbManager.getDevices 接口返回数据集合，取其中一个设备对象，并获取权限 。
//把获取到的设备对象作为参数传入usbManager.connectDevice;当usbManager.connectDevice接口成功返回之后；
//才可以调用第三个接口usbManager.claimInterface.当usbManager.claimInterface 调用成功以后,再调用该接口。
let devicesList: Array<usbManager.USBDevice> = usbManager.getDevices();
if (devicesList.length == 0) {
  console.log(`device list is empty`);
}

let device: usbManager.USBDevice = devicesList[0];
usbManager.requestRight(device.name);

let devicepipe: usbManager.USBDevicePipe = usbManager.connectDevice(device);
for (let i = 0; i < device.configs[0].interfaces.length; i++) {
  if (device.configs[0].interfaces[i].endpoints[0].attributes == 2) {
    let endpoint: usbManager.USBEndpoint = device.configs[0].interfaces[i].endpoints[0];
    let interfaces: usbManager.USBInterface = device.configs[0].interfaces[i];
    let ret: number = usbManager.claimInterface(devicepipe, interfaces);
    let buffer =  new Uint8Array(128);
    usbManager.bulkTransfer(devicepipe, endpoint, buffer).then((ret: number) => {
      console.log(`bulkTransfer = ${ret}`);
    });
  }
}