You can’t use GET like that. In your case you need to create a hidden input in the form and bind the session value to it. Then use the {{$_POST.hidden_field}} in the dynamic path.
@Teodor
It works perfect as long as I upload an image. But if I dont want to add an image and press “next”
I get an error.
When I try to look at the error in firefox developertools I get this:
SyntaxError: JSON.parse: unexpected non-whitespace character after JSON data at line 1 column 1039 of the JSON data
Well if you don’t select an image the upload step receives a null value, which causes the error.
Just wrap your upload step in a condition and just use {{$_POST.your_input_here}} as an expression. If the input has a value (file is selected) the upload placed in the then step will run, else it won’t.