BUG: Fixed fancybox_preview when using ref_urls plugin.

* Fancybox could not determine the type of the URL automatically, and
  showed garbage.
* We now use the new custom options hook (you'll need r246 from
  resource-plugins for this) to detect the image type by extension.


git-svn-id: http://svn.montala.net/svn/resourcespace@5056 c08608d7-6e46-0410-86ca-f2a6f1370df5