To those suggesting he took the briefcase, etc with him on the jump and SURVIVED are ignoring the physics of skydiving.
Any asymmetry in a jump will put you into a spin. If you aren't symmetrical to the air flow in a jump, you will start turning, flipping, and spinning. If Cooper is jumping with his main chute, a jerry-rigged decoy chute packed with cash, the knapsack, AND the briefcase, then he is putting himself in serious risk of an uncontrolled freefall leading to death.
I personally believe Cooper died in the jump, so him going into an irreversible spin before lawndarting to Earth is fine with me, but for those who believe he survived, he would have needed to keep his person as symmetrical as possible.