HarmonyOS 鸿蒙Next文件下载后文件夹内找不到对应文件

1. 文件下载，下载的是应用的沙箱目录，沙箱目录参考文档： https://developer.huawei.com/consumer/cn/doc/harmonyos-guides-V5/app-sandbox-directory-V5#应用沙箱目录与应用沙箱路径

   IDE右下角Device File Browser可以查看沙箱目录下的文件

2. 沙箱文件保存到文件管理器参考：

import { common } from '@kit.AbilityKit';
import { BusinessError, request } from '@kit.BasicServicesKit';
import fs from '@ohos.file.fs';
import { buffer } from '@kit.ArkTS';
import { picker, storageStatistics } from '@kit.CoreFileKit';
import { webview } from '@kit.ArkWeb';

let context = this as common.UIAbilityContext;

@Entry
@Component
struct testPdfSave {
  testIndex: number = 0;
  build() {
    Column() {
      Button('保存').onClick(() => {
        let filePath = context.cacheDir + "/test" + this.testIndex + ".pdf";
        this.testIndex = this.testIndex + 1
        let url = "https://gitee.com/harmonyos-cases/cases/raw/master/CommonAppDevelopment/feature/webpdfviewer/src/main/resources/rawfile/sample.pdf"
        try {
          request.downloadFile(context, {
            url: url,
            filePath: filePath
          }).then((data: request.DownloadTask) => {
            let downloadTask: request.DownloadTask = data;

            downloadTask.on('complete', () => {
              console.info('download complete');
              let file = fs.openSync(filePath, fs.OpenMode.READ_WRITE | fs.OpenMode.CREATE);
              let arrayBuffer = new ArrayBuffer(4096000);
              let readLen = fs.readSync(file.fd, arrayBuffer);
              let buf = buffer.from(arrayBuffer, 0, readLen);
              console.info(`content of File: ${buf.toString()}`);
              let writeLen = fs.writeSync(file.fd, arrayBuffer);
              fs.closeSync(file);

              const documentSaveOptions = new picker.DocumentSaveOptions(); // 创建文件管理器选项实例
              documentSaveOptions.newFileNames = ["1.pdf"]; // 保存文件名（可选）
              documentSaveOptions.fileSuffixChoices = ['']; // 保存文件类型（可选）
              let uris: Array<string> = [];
              const documentViewPicker = new picker.DocumentViewPicker(); // 创建文件选择器实例
              documentViewPicker.save(documentSaveOptions).then((documentSaveResult: Array<string>) => {
                uris = documentSaveResult;
                console.info('documentViewPicker.save to file succeed and uris are:' + uris);
                let uri = uris[0];
                let file2 = fs.openSync(uri, fs.OpenMode.READ_WRITE | fs.OpenMode.CREATE);
                let writeLen2 = fs.writeSync(file2.fd, arrayBuffer);
                fs.closeSync(file2);
              }).catch((err: BusinessError) => {
                console.error(`Invoke documentViewPicker.save failed, code is ${err.code}, message is ${err.message}`);
              })
            })

            let failCallback = () => {
              console.info('Download task fail.');
            };
            downloadTask.on('fail', failCallback);

          }).catch((err: BusinessError) => {
            console.error(`downLoadFileTask failed, code is ${err.code}, message is ${err.message}`);
          });
        } catch (error) {
          console.error(`downLoadFileTask failed, code is ${error.code}, message is ${error.message}`);
        }

      })
    }
    .height('100%')
    .width('100%')
  }
}