Well, it’s not really Ajax uploading considering file upload with ajax is not possible, but the end result is similar.
This jQuery plugin creates a hidden iframe and sets the form’s target attribute to post to that iframe. When the form is submitted, it is posted (including the file uploads) to the hidden iframe. Finally, the plugin collects the server’s response from the iframe.
This pseudo ajax file upload plugin is about 550 bytes minified and gzipped.