gistfile1.txt

function! CompleteFiles(findstart, base)
    if a:findstart
        " Get cursor word.
        let cur_text = strpart(getline('.'), 0, col('.') - 1)

        return match(cur_text, '\f*$')
    endif

    echomsg printf('getline(".") = %s, a:base = %s',
          \ getline('.'), a:base)
    let words = split(expand(a:base . '*'), '\n')
    let list = []
    let cnt = 0
    for word in words
        call add(list, { 'word' : word, 'abbr' : printf('%3d: %s', cnt, word), 'menu' : 'file_complete' })
        let cnt += 1
    endfor

    return list
endfunction

setlocal omnifunc=CompleteFiles