I want to add (or update, if present) the path to the current file I am working on, at the top of that file.
For example, if I place File: near the top of the file I that I am editing in vim (neovim), I want to automatically update that line with the path and filename of the file that I am editing; e.g.
File: /mnt/Vancouver/this_file.sh
If it helps, I have the following in my .vimrc file that automatically adds the date after the Last modified: line (if present) near the top of my file, anytime I save that buffer.  (The cursor position is also automatically restored, via keepjumps.)
" http://vim.wikia.com/wiki/Insert_current_date_or_time 
" If buffer modified, update any 'Last modified: ' in the first 30 lines.
" 'Last modified: ' can have up to 10 characters before (they are retained).
" Restores cursor and window position using save_cursor variable.
function! LastModified()
  if &modified
    let save_cursor = getpos(".")
    let n = min([30, line("$")])
    keepjumps exe '1,' . n . 's/^\(^Last modified: \).*/\1' .
          \ strftime('%Y-%m-%d') . '/e'
    call histdel('search', -1)
    call setpos('.', save_cursor)
  endif
endfun
autocmd BufWritePre * call LastModified()
" TEST:
" Last updated: 
" (indented line below: should not update)
"  Last modified: 
" Last modified: 2018-11-21