title: Fix file format with QFileDialog.getSaveFileName in PyQt5
body: <p>I am creating an app with PyQt5. It opens a web-browser and it should allow the user to download files. </p>
  
  <p>It looks like this:</p>
  
  <pre><code>import sys
  from PyQt5 import QtWidgets,QtGui,QtCore
  from PyQt5.QtWebEngineWidgets import *
  from PyQt5 import QtWebChannel
  from PyQt5.QtWidgets import *
  
  
  def _downloadRequested(item): # QWebEngineDownloadItem
      options = QFileDialog.Options()
      options |= QFileDialog.DontUseNativeDialog
      #
      global pathsave_custom
      pathsave_custom = QFileDialog.getSaveFileName(None, "Select destination folder and file name", "", "Zip files (*.zip)",
                                  options=options)[0]
      print('downloading to', item.path())
      item.setPath(pathsave_custom)
      print('downloading to', item.path())
      print(item)
      print(type(item))
      item.accept()
  
  
  
  app=QtWidgets.QApplication(sys.argv)
  w=QWebEngineView()
  w.page().fullScreenRequested.connect(QWebEngineFullScreenRequest.accept)
  w.load(QtCore.QUrl('https://google.com'))
  w.page().profile().downloadRequested.connect(_downloadRequested)
  w.showMaximized()
  app.exec_()
  </code></pre>
  
  <p>I would like to impose the format of the downloaded file. No matter what the user writes, there should be an extension chosen by me at the end of the downloaded file. 
  How can I do that? </p>