need "All Files" option in DroneCAN fw update file selector

the dialog in DroneCAN fw update file selector needs an All Files option
this is needed for firmwares that use a different extension, such as the Currawong DroneCAN ESCs

Locate the DroneCAN firmware update file selector in the Mission Planner desktop UI and inspect how its file filters are defined. Add an All Files choice that permits firmware with nonstandard extensions, then verify that such files can be selected in the dialog.

Written by the indexing model from the issue text.