*.mm filetypes use non-existent 'objective-c' filetype

[6112]

Steps

:e foo.mm

Outcome

Syntax highlighting doesn't work. :echo %opt{filetype} echoes objective-c

On my machine, it looks like this extracted from file -b --mime-type -L "${kak_buffile}", which outputs text/x-objective-c.

Expected

Syntax highlighting works. :echo %opt{filetype} echoes objc

*.m files work fine

[6112]

There's currently no objcpp filetype that covers all the features of Objective-C++, but objc would be a good fallback if we just want a quick "fix"

We may want to add an objcpp filetype eventually, that can handle all the indentation, highlighting, etc. that's C++-specific