在新的智能手机上无法下载

Question

我的应用程序在android API 24及更低版本上运行良好，但在API 25及更高版本上存在下载问题！

首次打开该应用程序时，会显示通知以从Internet下载文件。该问题发生在API 25及更高版本上，显示了通知，但文件未下载。

我试图通过将API级别和targetSdkVersion更新为28来修复它，我也查看了使用许可，但没有成功。

我需要你的建议。

Public class MainActivity extends Activity {
ImageView m,m2;
ImageButton v1,v2,v3,v4,v5;
private long backPressedTime = 0;
private ProgressDialog mProgressDialog;
final Context context = this;

 String Url="https://www.dropbox.com/s/abc=1";
 String unzipLocation = Environment.getExternalStorageDirectory() + "/QFolder/";
 String StorezipFileLocation =Environment.getExternalStorageDirectory() + "/DownloadedZip"; 
 String DirectoryName=Environment.getExternalStorageDirectory() + "/QFolder/files/";

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

     File f = new File(Environment.getExternalStorageDirectory()+"/QFolder/files/");
        if(f.exists() == false)
        {
            AlertDialog.Builder alertDialogBuilder = new AlertDialog.Builder(context);

                alertDialogBuilder.setTitle("Download Now");


                alertDialogBuilder
                    .setMessage(" File Size 75 MB")
                    .setCancelable(false)
                    .setNegativeButton("äÚã",new DialogInterface.OnClickListener() {
                        public void onClick(DialogInterface dialog,int id) {
                            new DownloadZipfile();
                              mew.execute(Url);
                        }
                      })
                    .setPositiveButton("..",new DialogInterface.OnClickListener() {
                        public void onClick(DialogInterface dialog,int id) {

                            dialog.cancel();
                        }
                    });
                    // create alert dialog
                    AlertDialog alertDialog = alertDialogBuilder.create();
                    // show it
                    alertDialog.show();
        }
    }
}