好像下载这个apk文件需要专门的headers，代码如下：

public function download(Request $request)
{
    $headers = [
        'Content-Type'=>'application/vnd.android.package-archive',
        'Content-Disposition'=> 'attachment; filename="android.apk"',
    ];
    return response()->download(public_path('apk/app.apk'), 'app.apk', $headers);
}