The following lines:

QFileDialog dir_selector(this);
dir_selector.setFileMode(QFileDialog::Directory);
dir_selector.exec()

Correctly execute a Windows-native directory explorer, but does not display the files inside, contrary to the documentation:

QFileDialog::Directory 2 The name of a directory. Both files and directories are displayed.

It should be noted that the output of:
qDebug() << dir_selector.testOption(QFileDialog::ShowDirsOnly);
is false.