/usr/share/vim/vim80/ftplugin/tmux.vim is in vim-runtime 2:8.0.1453-1ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 | " Vim filetype plugin file
" Language: tmux(1) configuration file
" URL: https://github.com/ericpruitt/tmux.vim/
" Maintainer: Eric Pruitt <eric.pruitt@gmail.com>
" Last Changed: 2017 Mar 10
if exists("b:did_ftplugin")
finish
endif
let b:did_ftplugin = 1
setlocal commentstring=#\ %s
|