Google Docs's drag & drop file upload. How is it possible?

Google docs now supports drag & drop upload.

How did they do? How did they implement progressbar for uploading file? Did they use HTML5 websocket? If I use XMLHttpRequest for the uplaod, is there a way开发者_如何学编程 to show a progressbar to user?

By the way, a folder still can not be upload in Google docs yet. Is it impossible?

---

Yes, it's a HTML5 feature.