function upload_delete_revision($node) {
  if (is_array($node->files)) {
    $files = array();
    foreach ($node->files as $file) {
      // Check if the file will be used after this revision is deleted
      $count = db_result(db_query('SELECT COUNT(fid) FROM {upload} WHERE fid = %d', $file->fid));

      // if the file won't be used, delete it
      if ($count < 2) {
        drupal_delete_add_query('DELETE FROM {files} WHERE fid = %d', $file->fid);
        $files[$file->fid] = $file;
      }
    }
  }

  // Delete the revision.
  drupal_delete_add_query('DELETE FROM {upload} WHERE vid = %d', $node->vid);

  // Register a callback to delete the files.
  drupal_delete_add_callback(array('upload_delete_post' => array($files)));
}

function upload_delete_post($files) {
  foreach ($files as $file) {
    // Delete all files associated with the node or revision.
    file_delete($file->filepath);
  }
}