Hi Fabian,
unfortunately, there are some elements on the form which are not available, but there are workarounds. :)
In your case I would create a form rule of type JavaScript mode. Add a parameter, and write some JavaScript to hide/ show the attachment element.
Afterwards you can use this form rule in your current one and you should be good. :)
Here's an example of the same approach though the JavaScript is doing something else.
https://community.webcon.com/forum/thread/552/601
Off topic:
I would add a constant for the GUID, even if the GUID itself is a constant. :)
This ways you will be able to utilize the "Usage" tab in a few month, when you will have forgotten where the GUID is used. Ok, I'm not sure whether you will forget it but I'm quite sure that I would have forgotten it. Hm, I think englisch wording isn't correct but I think you will get the meaning. :)
Best regards,
Daniel