I may have misunderstood your intention so if I have I apologize in advance! As I understand it, you want to sort the List your attachments are stored in and keep it sorted the same way between user steps. You could do this one of two ways but both revolve around the creation of an algorithm with which to do so:
1) Using SetValues, you can build an algorithm that sorts your List in the way you prefer - such as alphabetically.
2) Using an Execute Script object, you could write a script that runs your List variable through a sort algorithm.
Once again, I may have misunderstood your point. Either way, please let me know if you have any questions.