This isn't very pretty, but it will work. Just pass all your form elements and populate hidden elements on your next page's form. You final form will have all the data you require.
Not only is it not pretty, it's also a security hole the size of the Titanic

If you want form1 to have the upload page, then you should process the file upload on page2. That's about as simple as it gets and as simple as I can make it. Store it in some semi-temporary directory on your host (preferably outside the server software's scope), and continue with the rest of the pages. Then on the last page, process the rest of the data, and apply whatever you need to, to the previously uploaded file.
PHP says: if you don't process the file -right away-, I'll get rid of it, no questions asked.