class MainWindow(QtWidgets.QDialog):
    def __init__(self):
       super(MainWindow, self).__init__()
       self.setWindowTitle("My own MainWindow")

       self.fileDialog = QtWidgets.QFileDialog(self)
       self.fileDialog.setWindowFlags(Qt.Widget)

       self.confirmAction = QtWidgets.QPushButton("Press me", self)

       mainLayout = QtWidgets.QHBoxLayout()

       mainLayout.addWidget(self.fileDialog)
       mainLayout.addWidget(self.confirmAction)
       self.setLayout(mainLayout)