DroneCAN/UAVCAN Menu, Update should support .hex files for AM32 DroneCAN ESCs

ArduPilot has support the AM32 DroneCAN ESCs for about a year and I've noticed that it is not possible to use Mission Planner's DroneCAN/UAVCAN screen to update the firmware because:

- the am32 firmware site ([https://am32.ca/downloads](https://am32.ca/downloads)) only provides .hex files
- MP's Update screen only supports .apj or .bin files

By the way, I did try just uploading the .hex but it resulted in the ESCs becoming unuable with messages including "bad eeprom header" and "no app signature" and I had to reload the bootloader

FYI @Huibean

Start at Mission Planner's DroneCAN/UAVCAN Update screen and trace where firmware file extensions are validated, alongside the existing .apj and .bin handling. Done means the screen safely accepts the .hex files provided for AM32 DroneCAN ESCs and updates them without the bootloader or application-signature errors described.

Written by the indexing model from the issue text.