FileSystemException：尝试写入新文件时无法打开文件

Question

我正在尝试使用自定义名称编写文件，但是要遇到错误，但是如果我使用常数名称，则可以正常工作，

这是我的代码

    Future downloadFile(String url, String name) async {
//creating path
final tempDir =
    (await AndroidExternalStorage.getExternalStoragePublicDirectory(
        DirType.downloadDirectory));
//filename genearted from 
customname = generated;
final file = File('$tempDir/myfolder/$customname');
  try {
    final response = await Dio().get(url,
        onReceiveProgress: (count, total) {
      setState(() {
        isDownloading = true;
        progress = ((count / total) * 100);
        if (progress == 100) {
          isDownloaded = true;
        }
      });
    },
        options:
            Options(responseType: ResponseType.bytes, receiveTimeout: 0));

    var raf = file.openSync(mode: FileMode.write);
    raf.writeByteSync(response.data);
    await raf.close();
  } catch (e) {
    log(e.toString());
  }

}

错误日志

[log] FileSystemException: Cannot open file, path = '/storage/emulated/0/Download/myfolder/dunkin.zip' (OS Error: Permission denied, errno = 13)