Cooper used shroud line to create a belt around his waist. I believe he used this belt to attach the bank bag, the dummy reserve (holding 50K), and the attache' case. This means everything Cooper would have been jumping with was attached to this belt made of shroud line and the items would have been hanging in front of his crotch and thighs just like military jumpers and smoke jumpers.
Do you have a theory as to why he would take everything with him? For instance, if all the money fit into the bag and the reserve, is there a reason why he would take the briefcase with him as opposed to just pitching it out the back?
Would all the money (or how much of it would) fit into the briefcase if he pitched the 'bomb' contents?
The overall point being, if all the money fit into one or two of those containers, is there a reason he would encumber himself with all three items?
Simply put, the attache' case is evidence.
The attache' case was reported as looking inexpensive, but new. Therefore, it reasonable to deduce that DBC considered it a treasure trove of evidence if the authorities were to get a hold of it.
I believe that the attache' case walked out of the drop zone with the money.