title: Can't add extension to file in QFileDialog
body: <p>I've got a problem with saving file with extension (get path to file and append extension) in PyQt4 with QFileDialog. My Python code looks like that:</p>
  
  <pre><code>dialog = QtGui.QFileDialog()
  dialog.setDefaultSuffix(".json")
  file = dialog.getSaveFileName(None, "Title", "", "JSON (.json)")
  </code></pre>
  
  <p>It works, path is correct, dialog title and filter are in dialog window, but second line was ignored. File doesn't have any extension.</p>
  
  <p>How to add extension by default? What am I doing wrong?</p>