File Uploads As Custom Field

I was wondering if you could either do a quick update or tell me how I could edit CustomPress to allow for a custom field with a file upload? This is a critical part of my project so it would be greatly appreciated.

I found this: http://codex.wordpress.org/Plugin_API/Filter_Reference/post_edit_form_tag

I would /assume/ it's not too difficult, if you could do this it would be amazing. thank you.