My article "New Tricks in XMLHttpRequest2" has many fine examples, but what it doesn't have is any server code to illustrate how to handle files. If you're curious how to process a file upload using xhr.send(FormData), here's a quick example of an image upload in PHP.
This server is trivial but it demonstrates two things. The first is sending a file and extra payload at the same time. The second is how to grab the file (and other data) in PHP. Lastly, the image is encoded into a data: URL and included in a JSON response sent back to the client.