Add mtime to files in Drupal

add-mtime-file-url.php

<?php

/**
 * Implements hook_file_url_alter().
 */
function HOOK_file_url_alter(&$uri) {

  // Client is complaining about images not refreshing immediately when the user
  // changes one image for another (and the filename stays the same). Using the
  // image path flush function doesn't trigger a browser cache refresh for the
  // image, so we adjust all image URLs to include the last modified time of
  // the image.

  $processed_uris = &drupal_static(__FUNCTION__, array());

  // Only check URLs with the following extensions.
  $process_exts = array('gif', 'jpg', 'jpeg', 'png');
  $ext = pathinfo($uri, PATHINFO_EXTENSION);
  $has_ext = in_array(strtolower($ext), $process_exts);

  // If the extension is one of the preset image types and not already
  // processed.
  if ($has_ext && !in_array($uri, $processed_uris)) {

    // Add this URI to our processed list.
    $processed_uris[] = $uri;

    // And finally create the new URL with a timestamp.
    $uri = file_create_url($uri) . '?mt=' . filemtime(drupal_realpath($uri));
  }
}