build right args in attachment builder

app/builders/learning_object/attachment_builder.rb

@@ -8,6 +8,8 @@ module LearningObject
     end
 
     def self.build_from_orientdb(args = {})
+      args['retrieve_link'] = args.delete('retrieveLink')
+      args['mime_type'] = args.delete('mimeType')
       build_from_cache args do
         ::LearningObject::Attachment.new(args)
       end
@@ -26,5 +28,12 @@ module LearningObject
       object
     end
 
+    def self.permitted_args(args={})
+      permitted = [:id, :name, :link, :retrieve_link, :description, :format, :mime_type, :size]
+      args.delete_if do |key, value|
+        !permitted.include? key.to_sym
+      end
+    end
+
   end
 end
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