Currently, only Firefox 3.6+ has successfully implemented the above. The latest Webkit(Chrome/Safari) support file upload, but only a single file at a time as outlined by Andrea Giammarchi. The rest of the browsers have no support for file upload over an XHR Request.
There are 3 different ways to send files without refreshing the page, native upload using the FileReader API, single file upload using the upload XHR Request, and submitting a form through an iframe. This script handles all 3, but with limitations. A normalize method was extended onto the Upload constructor at an attempt to keep things consistent. What this does is force all browsers that don't have the FileReader API implemented to use the frame hack.
You can enable file upload by calling the Upload.denormalize method, but this will cause inconsistency in the way file uploads are handled. Mainly in that you can no longer send multiple files, and can only send data as GET parameters, instead of POST. Still, with it enabled, File drag and drop from desktop will be supported.
IE 6, 7, 8
FireFox 126.96.36.199, 3.0.17, 3.5.7, 3.6
Safari 3.0.4, 3.1.2, 3.2.1, 4.0.5, 5.0
Opera 9.52, 9.64, 10.01, 10.10
Andrew Valums - Original Author
Andrea Giammarchi - Chrome/Safari Hack
Corey Hart - Creator of this adaptation