code: function registerPostMessageCallback(iframe, callback) {
          var iframeName = iframe.id,
              fileId = getFileIdForIframeName(iframeName),
              uuid = getUuid(fileId);
  
          onloadCallbacks[uuid] = callback;
  
          // When the iframe has loaded (after the server responds to an upload request)
          // declare the attempt a failure if we don't receive a valid message shortly after the response comes in.
          detachLoadEvents[fileId] = qq(iframe).attach("load", function() {
              if (handler.getInput(fileId)) {
                  log("Received iframe load event for CORS upload request (iframe name " + iframeName + ")");
  
                  postMessageCallbackTimers[iframeName] = setTimeout(function() {
                      var errorMessage = "No valid message received from loaded iframe for iframe name " + iframeName;
                      log(errorMessage, "error");
                      callback({
                          error: errorMessage
                      });
                  }, 1000);
              }
          });
  
          // Listen for messages coming from this iframe.  When a message has been received, cancel the timer
          // that declares the upload a failure if a message is not received within a reasonable amount of time.
          corsMessageReceiver.receiveMessage(iframeName, function(message) {
              log("Received the following window message: '" + message + "'");
              var fileId = getFileIdForIframeName(iframeName),
                  response = handler._parseJsonResponse(message),
                  uuid = response.uuid,
                  onloadCallback;
  
              if (uuid && onloadCallbacks[uuid]) {
                  log("Handling response for iframe name " + iframeName);
                  clearTimeout(postMessageCallbackTimers[iframeName]);
                  delete postMessageCallbackTimers[iframeName];
  
                  handler._detachLoadEvent(iframeName);
  
                  onloadCallback = onloadCallbacks[uuid];
  
                  delete onloadCallbacks[uuid];
                  corsMessageReceiver.stopReceivingMessages(iframeName);
                  onloadCallback(response);
              }
              else if (!uuid) {
                  log("'" + message + "' does not contain a UUID - ignoring.");
              }
          });
      }
docs: If we are in CORS mode, we must listen for messages (containing the server response) from the associated
  iframe, since we cannot directly parse the content of the iframe due to cross-origin restrictions.
  
  @param iframe Listen for messages on this iframe.
  @param callback Invoke this callback with the message from the iframe.