Taxonomy plugin does not work for ImagesSet types

When adding the Classifiable plugin to an Imageset type, the plugin does not work and it does not give any useful feedback.
To fix this issue it requires a manual change in the CND file, by adding a hippotaxonomy:classifiable to the imageset type.

Ex:

 [demo:imageset] > hippotaxonomy:classifiable, hippogallery:imageset, hippogallery:relaxed
  orderable