I have a web page that opens a div when you click a button. This div allows you to drag a file from your desktop onto its area; the file then gets uploaded to the server. I\
You can use Blueduck Sda (http://sda.blueducktesting.com) Is an OSS that has implemented ALL selenium functions (It works with selenium RC) but it allows you to automate Windows actions. So you can test web, and interact with the OS. So you can make your test, and then, just tell the mouse to click on the element and drop it where you want!
Nice testing!