Add media_klass to filter class by media_type

lib/whaleback/media/obj.rb

@@ -36,7 +36,17 @@ module Whaleback
         'video'    => 'Media::Video',
         'voice'    => 'Media::Voice',
       }
-      
+
+      class << self
+        def media_klass media_type
+          Media::Obj::TypeTrans[ media_type.to_s ].to_s.safe_constantize || raise_wrong_media_type(media_type)        
+        end
+
+        def raise_wrong_media_type
+          raise Whaleback::Errors::BaseError.new("Invalid media_type: #{media_type}", -1, 400)          
+        end
+      end
+
       attr_accessor :default_mode
       
       def url mode=nil