You can’t upload files using AJAX, you’ll have to use one of the many workarounds that are available (use google to search for “ajax upload”), but here’s a few options you might consider:
• mongrel_upload_progress (http://svn.techno-weenie.net/projects/mongrel_upload_progress/)
• SWFUpload (http://swfupload.mammon.se/): uses Flash for the upload, but works really great
Best regards
Peter De Berdt