Comment from Steven: It does this by redirecting the submission of the form to a hidden <iframe> when you click "Attach" (we cannot submit data through Ajax directly because you cannot read file contents from JS for security reasons).  Once the file is submitted, the upload-section of the form is updated.  Things to note:

    * The feature degrades back to the current behaviour without JS.
    * If there are errors with the uploaded file (disallowed type, too big, ...), they are displayed at the top of the file attachments fieldset.
    * Though the hidden-iframe method sounds dirty, it's quite compact and is 100% implemented in .js files. The drupal.js api makes it a snap to use.
    * I included some minor improvements to the Drupal JS API and code.
    * I added an API drupal_call_js() to bridge the PHP/JS gap: it takes a function name and arguments, and outputs a <script> tag. The kicker is that it preserves the structure and type of arguments, so e.g. PHP associative arrays end up as objects in JS.
    * I also included a progressbar widget that I wrote for drumm's ongoing update.php work. It includes Ajax status updating/monitoring, but it is only used as a pure throbber in this patch. But as the code was already written and is going to be used in the near future, I left that part in. It's pretty small ;). If PHP supports ad-hoc upload info in the future like Ruby on Rails, we can implement that in 5 minutes.