By this snippet you can list/index a specific path with specific extension(s) to a text file. (Indexing duration depends on amount of folders/files you have and your PC speed.)

Usage: /findex [-nx] [Extension]
Example: /findex -nx *.mp3 D:Music

[-nx] properties and [Extension(s)] are optional.
n: To exclude the path which you enter with your command.
x: To index a specific file extension(s).
You can specify one or more file extension(s) to index them. (.txt;.jpg, etc... Separating by semicolon.)

You can simply copy/paste the code into your remote or downloading the .mrc file from HERE

Updated

• There was a bug that it didn't index the files which are in the first root directory.
• Now it supports multi extensions.

alias FIndex {
  ; The [-nx] and [Extension(s)] properties are optional.
  ; n: Use it if you want to exclude the path which you enter with your command.
  ; x: Use it if you want to index a specific file extension.
  ; You can specify one or more file extension(s) to index them. (*.txt;*.jpg, etc... Separating by semicolon.)
  var %path = $iif($left($1,1) == -,$iif(x isin $1,$3-,$2-),$1-)
  set %IndexerSPath $sdir($mircdir,Select a path to save your output file)
  set %IndexCount 0
  if (n isin $1) { set %IndexerNoPath %path }
  else { unset %IndexerNoPath }
  if ($isfile($qt(%IndexerSPath $+ Indexer.txt))) { 
    var %msg = The file is already exists! Do you want to replace it? $+ $crlf $+ YES $+ $chr(9) $+ = Replace the file. $+ $crlf $+ NO $+ $chr(9) $+ = Append the new info into the file. $+ $crlf $+ CANCEL $+ $chr(9) $+ = Do nothing, just return.
    var %q = $input(%msg,nvgq,File is already exists...)
    if (%q == $yes) { .write -c $+(",%IndexerSPath,Indexer.txt,") }
    elseif (%q == $no) { }
    elseif (%q == $cancel) return
  }
  if (!$1) || (!$isdir($qt(%path))) || (!%IndexerSPath) { echo $color(info) -a *** File Indexer Syntax: /findex [-nx] [Extension] <Path> | return }
  var %ext = *.*
  if ($left($1,1) == -) && (x isin $1) && (*.* iswm $2) { %ext = $2 }
  mWriteInfo -i Path: $qt(%path) $+ $crlf $+ Extension(s): $qt(%ext)
  set %IndexTime $ticks
  .noop $findfile(%path,%ext,0,0,mWriteInfo $1-)
  var %a = 1,%b = $finddir(%path,*.*,0)
  while (%a <= %b) {
    .noop $findfile($finddir(%path,*.*,%a),%ext,0,0,mWriteInfo $1-)
    inc %a
  }
  mWriteInfo -i Operation succeed in: $calc($calc($ticks - %IndexTime) / 1000) ms. $+ $crlf $+ Indexed files: %IndexCount $+ $crlf $+ E.O.F.
  echo $color(info) -a *** Operation succeed and File has been saved to $+(",%IndexerSPath,Indexer.txt,")
  unset %Index*
}

alias -l mWriteInfo {
  if (!$1) || (!%IndexerSPath) return
  if ($1 == -i) {
    .write $+(",%IndexerSPath,Indexer.txt,") $str(-,40)
    .write $+(",%IndexerSPath,Indexer.txt,") $2-
    .write $+(",%IndexerSPath,Indexer.txt,") $str(-,40)
  }
  else {
    if ($len(%IndexerNoPath) > 0) && ($right(%IndexerNoPath,1) != \) { set %IndexerNoPath $+(%IndexerNoPath,\) }
    .write $+(",%IndexerSPath,Indexer.txt,") $remove($1-,%IndexerNoPath)
    inc %IndexCount 1
  }
}