Briefcases (specifically) don't come with straps--you'd hold it in your hand/wield the briefcase.  Since that both disables combat and doesn't increase your capacity, I'd recommend encumbering Arms instead--shooting or throwing would effectively be one-handed whilst you're holding The Bag, and it might mess up your balance for throwing.  (If you've ever played TimeSplitters multiplayer, they do an OK job of this.)