PHP AJAX example: asynchronous file upload

Asynchronous file uploads is quote popular feature in modern AJAX web-applications. However standard AJAX classes (XmlHttpRequest) does not have capabilities to process or send files selected with "file dialog" (input type="file"). This article contains example application (trivial file-sharing service, like rapidshare, megaupload or yousendit) which uses embedded frames (IFRAME) to upload file. While file is uploaded to hidden frame, user can still access web-page and fill "file description" field.