It's very simple.
The file gets uploaded after page one is submitted. You then need to move the uploaded file from the temporarely location to a location to store it until the mail is sent. All the code you need for this is inside the example in the link i posted.
At that stage, you have the fileadress, and you then just pass along the fileadress, just like the other values. The file itself just sits n the directory you moved it to.
When you build the mail, you then attach the file to is using the fileadress.
If your not planning on learning to code this, then download a ready made class that does this all for you. Just google for 'PHP upload mail' or take a look at the usercomments in the link i posted or at www.hotscripts.com