Actually, looking at the code you posted changes the process I would suggest you use considerably.
First of all, the data the form collects is passed to a fairly sophisticated commercial formmailer script, which has it's own way of handling the data and fields. My advice was "generic", and more applicable to a situation where you were coding your own formmail application.
I took a quick look at the documentation for formprocessorpro on their website, it looks as though that program offers you the functionality you need, as it can handle multipart forms, and is highly configurable. For example, this excerpt:
...shows a couple of different ways you could approach this. I think coding your own conditional "switch" statement is probably the "hard" way to do this, and fails to use the power and "features" of the script you purchased. While learning this script's usage might seem difficult, I think that is where you have to start; jumping in and trying to modify it's functionality without thoroughly understanding it is pretty much guaranteed to be a formula for "grief".
If you tackle this problem the way you are envisioning it, you will need to build the form in a PHP page and use PHP, with the switch statement, to manipulate the values in the _redirect variable expected by the formprocessorpro script,or use PHP in the _redirect page to selectively display text (as we already discussed) rather than using the a raw HTML form, and using the features of the formprocessorpro script to accomplish your goal.
I suggest you contact support at formprocessorpro's website and ask them the best way to accomplish this using their script. Consider you will likely have better success with this if you do not presume how you might accomplish your goal, but rather let them suggest to you how you should do it. It doesn't make sense to "reverse-engineer" the script to implement a workaround that may not be necessary, and as a paying customer you are entitled to their support. They also advertise "customization services", though I think the type of information you will need to make this script do what you need will entail less customization than instruction.
I can guarantee you that they know that script a lot better than can I "grok" it with a quick look, and their advice is, consequently, likely to be of much higher quality than mine. I would hate to "kludge" together something "quick and dirty" to"hack" your form (I think giving "bad" or "inappropriate" advice or help is worse than giving none at all!), when the program probably has the functionality you need (once you fully understand it's workings). Unfortunately, I don't have the time right now to dig deeply enough into that formprocessingpro program to give you "better" advice. Maybe someone else here can do a better job with that than I can right now.
Please understand that I am not "blowing you off" on this; I just really think that is the way you should go. Of course, if that doesn't work for you, post back and I'll try to find some time to look at it further, but I have no guarantee when that might be.