An Airbus A319-111 aircraft, registration G-EZIX and operated by EasyJet Airlines, encountered turbulence during the flight that seriously injured one light attendant (FA). The flight continued and landed without further incident. During the descent before landing, and after passing through FL150, it crossed a layer of cumulus clouds some 2,000 ft thick that caused the aircraft to fall and shake sharply. As a result, the four light attendants, who were securing the passenger cabin, fell to the loor, receiving bruises in the process. One of the light attendants broke the scaphoid bone in her wrist while remained in the rear galley of the passenger cabin. The report identifies the cause of the accident as encountering strong turbulence that was not detected by the light crew