Ionic 4: After Image download success then image not showing in image gallary

Ionic: I am downloading images using FileTransfer.download() it a success, but it won’t show in the
image gallery unless not relaunch the image gallery app.

this.baseDownloadDir = ${this.file.externalRootDirectory}${this.userService.appName}/`;
downloadImage(src: string): void {
    if (!this.userService.getNetworkState()) {
      this.loaderService.showToaster(this.translateService.instant('ERROR_MESSAGES.NO_NETWORK'));
    } else {
      if (src) {
        const fileName = src.split('/');
        let imgName = fileName[fileName.length - 1];
        imgName = imgName.replace('.', `${new Date().getTime().toString()}.`);
        const downloadPath = this.baseDownloadDir + imgName;
        this.fileTransfer = this.transfer.create();
        this.diagnostic.isExternalStorageAuthorized().then((isAuthorized) => {
          if (isAuthorized) {
            this.fileDownload(src, downloadPath);
          } else {
            this.diagnostic.requestExternalStorageAuthorization().then((status) => {
              if (status === 'GRANTED') {
                this.fileDownload(src, downloadPath);
              }
            });
          }
        });
      }
    }
  }

  fileDownload(src, downloadPath) {
    this.fileTransfer.download(src, downloadPath).then((FileEntry) => {
      this.loaderService.showToaster(this.translateService.instant('ERROR_MESSAGES.DOWNLOAD_SUCCESS'));
    });
  }