AR Script: use the name of the first file in all others

  // Number of characters for the counter, padded with zeros if needed
  ZERO_PADDING = 3;

  if (index != 0){  return app.getItem(0).name + "_" + zpad(index.toString(), ZERO_PADDING); }

// Function that add leading zeros to any string
function zpad(str, nb_digits) {
    if (str.length < nb_digits)     { return (Array(nb_digits - str.length + 1).join('0') + str); }
    else                            { return str; }
}