Use selected text to fill initial query on popup

[albfan]

I search around for a working example and didn't find anything

function! FzfFilesWithVisualPrefilter() range
  let [lnum1, col1] = getpos("'<")[1:2]
  let [lnum2, col2] = getpos("'>")[1:2]
  let lines = getline(lnum1, lnum2)
  let lines[-1] = lines[-1][: col2 - (&selection == 'inclusive' ? 1 : 2)]
  let lines[0] = lines[0][col1 - 1:]

  let prefilter = lines[0]
  call fzf#vim#files('', fzf#vim#with_preview({ 'options': '--query "' . prefilter . '"'}))
  "call fzf#vim#files('', fzf#vim#with_preview({'options': {'query': prefilter}}))
endfunction

xnoremap <silent> <leader>ff :<C-U>call FzfFilesWithVisualPrefilter()<CR>

select a text, press ,ff and you have a popup searching for files matching your selected text.

Wonder if:

Is there a helper to get selected text for fzf.vim?

Should a line like:

call fzf#vim#files('', fzf#vim#with_preview({'options': {'query': prefilter}}))

work. Searching for a cleaner way to compose --query line