I think you are on the same boat with us:
http://wordpress.org/extend/ideas/topic/custom-attachment-type
Its not possible easily at the moment. If you and others supports my idea, maybe we will see it for wordpress 3.7
I think you are on the same boat with us:
http://wordpress.org/extend/ideas/topic/custom-attachment-type
Its not possible easily at the moment. If you and others supports my idea, maybe we will see it for wordpress 3.7