code: function(controller, url) {
    controller.open(url);
  
    // Wait until the unknown content type dialog has been opened
    controller.waitForEval("subject.getMostRecentWindow('').document.documentElement.id == 'unknownContentType'",
                           gTimeout, 100, mozmill.wm);
  
    utils.handleWindow("type", "", function (controller) {
      // Select to save the file directly
      var saveFile = new elementslib.ID(controller.window.document, "save");
      controller.waitThenClick(saveFile, gTimeout);
      controller.waitForEval("subject.selected == true", gTimeout, 100,
                             saveFile.getNode());
    
      // Wait until the OK button has been enabled and click on it
      var button = new elementslib.Lookup(controller.window.document,
                                          '/id("unknownContentType")/anon({"anonid":"buttons"})/{"dlgtype":"accept"}');
      controller.waitForElement(button, gTimeout);
      controller.waitForEval("subject.okButton.hasAttribute('disabled') == false", gTimeout, 100,
                             {okButton: button.getNode()});
      controller.click(button);
    });
  }
docs: Download the file of unkown type from the given location by saving it
  automatically to disk
  
  @param {MozMillController} controller
  MozMillController of the browser window
  @param {string} url
  URL of the file which has to be downloaded