The CDN always prefixes the url [here](https://github.com/sonata-project/SonataMediaBundle/blob/master/CDN/Server.php#L31), and for a thumbnail delivered using imagine the url is already correct and should not be prefixed. Therefore I moved the logic to the ThumbnailFormat, and leave it to the ThumbnailFormat to use the CDN or not.

LiipImagine recently added CacheResolvers that allow to configure the CDN for the generated thumbnails. So you can do pretty advanced stuff combining the CDN for the reference file of the SonataMedia and using the LiipImagine CacheResolvers to configure the CDN for the thumbnails. Fe. upload the originals to the filesystem and put the thumbnails in the cloud. This was of course already possible if you did not use LiipImagine...