ffmpeg custom command: %in_file not substituted

ffmpeg custom command does not seems to work. %in_file and %out_file are not substituted.
The code in mm_ffmpeg.module is:

  if ($configuration['ffmpeg_video_custom']) {
    $options[] = sprintf($configuration['ffmpeg_video_custom_command'], $current_file, $output_file);
  }

It could be replaced with something like:

  if ($configuration['ffmpeg_video_custom']) {
    $str=str_replace("%in_file", $current_file, $configuration['ffmpeg_video_custom_command']);
    $options[] =str_replace("%out_file", $output_file, $str);
 }