Implementing hook_boost_storage_types()

I wanted to add an additional storage type to my boost cache. My module, Tournament Brackets, can generate an image of a tournament bracket as part of the default node view. If the bracket node is at say node/1234, then an image is generated at node/1234/bracket-image inside the node. Of course, the node html was cached perfectly by boost, but the image part was not. So, implemented hook_boost_storage_types() to add a PNG type:

function bracket_boost_storage_types() {

  $types = array();
  
  $types['image/png'] = array(
    'title'           => t('PNG'),
    'description'     => t('PNG output'),
    'extension'       => 'png',
    'enabled'         => FALSE,
    'gzip'            => FALSE,
    'lifetime_max'    => 3600,
    'lifetime_min'    => 0,
    'comment_start'  => '<!-- ',
    'comment_end'    => ' -->',
    'variants'        => array(),
  );

  return $types;  
}

And added the required .htaccess code (generated by boost) and the images were cached as well. So far, so good. The module improves the performance of the site tremendously.

I'm also using the expire module to automatically delete cached nodes when they are updated - and therein lies my problem. The node html was expired correctly on node updates, but the image part was not.

I browsed around in expire and found hook_expire_cache_alter() which allows the expired url list to be updated when a node is expired. Here's my implementation:

function bracket_expire_cache_alter(&$urls, $object_type, $object, $absolute_urls_passed) {

  if ($object_type != 'node' || !isset($object->type)) {
    return;
  }
  if ($object->type == 'bracket') {
    $urls['node-' . $object->nid . '-bracket-image'] = 'node/' . $object->nid . '/bracket-image';
  }
}

However, when the node is expired, boost wants to delete the file: .../node/1234/bracket-image_.html - which is incorrect - it should be .png. It looks like boost_expire_cache() uses the header info of the current page (html) to form the file extension.

In order for this to work, I ended up passing the file extension in bracket_expire_cache_alter():

  if ($object->type == 'bracket') {
    $urls['node-' . $object->nid . '-bracket-image'] = 'node/' . $object->nid . '/bracket-image_.png';
  }

as well as patching the boost_expire_cache() code to allow a url with an extension to be passed in and if so not appending the .html extension.

I was wondering if the module developers had a better solution - I feel like mine is sort of a kludge...

Thanks!