code: public String fileName(int index) {
          byte_vector v = fs.file_name(index).to_bytes();
          return Vectors.byte_vector2utf8(v);
      }
docs: Returns only the name of the file, whereas
  {@link #filePath(int)} returns the path (inside the torrent file) with
  the filename appended.
  
  @param index the file index
  @return the file name