title: Google Picker not selecting by file extension
body: <p>Given files in Drive with an (arbitrary) extension *.abc, this code...</p>
  
  <pre><code>gapi.load("picker", { "callback": function () {
      if (!picker) {
          var view = new google.picker.DocsView(google.picker.ViewId.DOCS);
  
          view.setMimeTypes("application/vnd.google.drive.ext-type.abc");
  
          view.setMode(google.picker.DocsViewMode.LIST);
          picker = new google.picker.PickerBuilder();
          picker.setTitle(TEXT.PICKER_PROMPT);
          picker.setAppId(CONST.APP_ID);
          picker.addView(view);
          picker.setOAuthToken(session.OAuthToken.access_token);
          picker.setCallback(pickerCallback);
          picker.setInitialView(view);
      };
      picker.build().setVisible(true);
  ));
  </code></pre>
  
  <p>...doesn't find any of the existing 'abc' files in drive. 
  These files are of mime type text/xml, and the following line DOES find them:</p>
  
  <pre><code>view.setMimeTypes("text/xml");
  </code></pre>
  
  <p>Why doesn't the search by extension work?</p>