QFile write fails when writing to external sdcard on android

#include <QCoreApplication>
#include <QFile>
#include <QTextStream>
#include <QDebug>

#ifdef __ANDROID__
#include <QtAndroid>
#endif

// Taken from https://bugreports.qt.io/browse/QTBUG-50759
bool check_permission() {
    QtAndroid::PermissionResult r = QtAndroid::checkPermission("android.permission.WRITE_EXTERNAL_STORAGE");
    if(r == QtAndroid::PermissionResult::Denied) {
        QtAndroid::requestPermissionsSync( QStringList() <<  android.permission.WRITE_EXTERNAL_STORAGE" );
        r = QtAndroid::checkPermission("android.permission.WRITE_EXTERNAL_STORAGE");
        if(r == QtAndroid::PermissionResult::Denied) {
            qDebug() << "Permission denied";
            return false;
        }
    }

    qDebug() << "Permissions granted!";
    return true;
}

int main(int argc, char *argv[]) {
    QCoreApplication a(argc, argv);

    if (!check_permission())
        return -1;

    QFile f("/storage/extSdCard/WriteTest.txt");
    if (!f.open(QIODevice::WriteOnly | QIODevice::Text)) {
        qDebug() << f.errorString();
        return -1;
    }

    QTextStream out(&f);
    out << "This is just a test\n";

    return a.exec();
}