Steps to reproduce / test case
When using QFileDialog::getSaveFileName, if you pass QFileDialog::HideNameFilterDetails as the flag the filter extensions are properly omitted but the default filter is not auto selected when the save dialog is shown. For example, say we used QFileDialog::getSaveFileName like so:

QString filename = defaultFilename;

filename = QFileDialog::getSaveFileName(this, tr("Foo"), filename,
filtersStr, &chosenFilterString,
QFileDialog::HideNameFilterDetails);

In this situation chosenFilterString is a QString that was automatically set to something like "PNG (*.png)". That filter is one of many (but not the first) filters in the filtersStr, separated by ;;. When the Save As dialog is shown the 1st filter, not the proper default filter, is shown. If you omit the QFileDialog::HideNameFilterDetails flag the correct default filter is shown. This may also effect other QFileDialog functions like getting a file to open etc.