There seem to be three choices but I don't know which is the 'right' one:
Arduino-COMBINED-dfu-usbserial-atmega16u2-Mega2560-Rev3.hex
Genuino-COMBINED-dfu-usbserial-atmega16u2-Mega2560-R3.hex
MEGA-dfu_and_usbserial_combined.hex
Maybe try each one?