Uploads attached to a product

I'm trying to attach an uploaded file to a product. The size of the file determines the price. I thought that adding a step in between the product_info page and the add_product action would work, but it doesn't. I put an include into extra_cart_actions, but the subsequent add_product action erases what I put into the session cart. Example:

Extra Cart Action file

switch ( action ) {
  case 'add_product':
  if (the session has no upload) 
    redirect to upload page
    exit
  else 
    break;  //this is where $_GET['action'] == 'add_product' overwrites what I put into $_SESSION['cart']->contents['product_id']
  

  case 'upload':
   attach to session // $_SESSION['cart']->contents['product_id']['filesize']
   redirect to add_product action
}

Any suggestions on how to better structure a file upload BEFORE the product enters the session? I don't want a product w/o an uploaded file to enter the session because then it will have 0 price... Maybe that isn't so bad... hmm.