[file:description] token working with 'Download link' format

yourmodule.tokens.inc

<?php

/**
 * @file
 * Token integration for the file_entity module.
 */

/**
 * Implements hook_token_info().
 */
function hook_token_info() {
  // File tokens.
  $info['tokens']['file']['description'] = array(
    'name' => t('File description'),
    'description' => t('The description of the file.'),
    'type' => 'file',
  );

  return $info;
}

/**
 * Implements hook_tokens().
 */
function hook_tokens($type, $tokens, array $data = array(), array $options = array()) {
  $replacements = array();

  $sanitize = !empty($options['sanitize']);

  // File tokens.
  if ($type == 'file' && !empty($data['file'])) {
    $file = $data['file'];

    foreach ($tokens as $name => $original) {
      switch ($name) {
        case 'description':
          $replacements[$original] = $sanitize ? check_plain($file->description) : $file->description;
          break;
      }
    }
  }

  return $replacements;
}