Set default external program for MIME or tag or other

[Zweibach]

Set default external program for MIME or tag or other.
If there's more than one program for a MIME or tag combination, get a dialogue when attempting to open to pick between. Not all files display equally well in all viewers for any given MIME and there's always edge-cases.

[bbappserver]

There are some technical limitations here which you might be able to bypass with some funky python library.
Normally you can just ask the operating system's workspace to open a file, and it decides what the default program for doing that is. If it is difficult to get an OS agnostic list of possible programs to open a file then an acceptable alternative would be an override in the settings, or list of overrides, that can be added using the system's file open dialog, or manually typing a path into a list.