1. Click "Attach a file" link
2. In place of the above link add a text box & browse button to browse for a local file. Below this add a link that says "Attach another file". Next to it, add a link that says "remove".
3. If they clidk the "Attach another file" link, add another file browser. If the click the remove link, remove the associated file browser.
4. Upload the files on submit using PHP.