file name returned by QFileDialog::getSaveFileName()

[mcosta]

QFileDialog::getSaveFileName() doesn't add extension to fileName this because you can select a "multiple extension filter".
For example

fileName = QFileDialog::getSaveFileName(this, tr("Open File"), "", tr("Image Files (*.png *.bmp *.jpg)"));

fileName = QFileDialog::getSaveFileName(this, tr("Open File"), "", tr("Image Files (*.png *.bmp *.jpg)"));

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Then you ave to specify the extension