It's not really Widgets and Dialogs, but ./plugins/platformthemes/gtk3/qgtk3dialoghelpers.cpp (i guess), but didn't know what to choose.

The problem is that when

QFileDialog::getSaveFileUrl( this, QString(), QUrl(), "KTuberling files (*.tuberling)" );

one gets a dialog like this

As you can see the .tuberling information is lost, so the user can't possibly save the file with the correct extension.