title: "All Supported" in "Files of type:" combobox of QFileDialog
body: <p>I made a subclass of QFileDialog and put some file formats in "Files of type" combobox. Expet from these file formats, I want to have an  "All Supported" item in this combobox. When some selects "All Supported", they will be able to select only the files of the supported formats.</p>
  
  <p>I have already looked at the cocept of QMimeDatabase but I am quite confused on how and where  can make the XML file, containing my custom formats. According to the documentation:</p>
  
  <p>"Applications which want to define custom MIME types need to install an XML file into the locations searched for MIME definitions. These locations can be queried with</p>
  
  <pre><code>QStandardPaths::locateAll(QStandardPaths::GenericDataLocation, QLatin1String("mime/packages"),
                            QStandardPaths::LocateDirectory);
  </code></pre>
  
  <p>"</p>
  
  <p>When I am calling locateAll, I get an empty QStringList.</p>
  
  <p>So my questions are:</p>
  
  <ol>
  <li><p>Do I really need the concept of some new entries in QMimeDatabase?</p></li>
  <li><p>If yes, where is the directory that I have to put my XML file?</p></li>
  </ol>