no fallar al serializar Site#everything_of

el problema era que los archivos subidos no se pueden marshalear y
estamos usando MetadataTemplate#values para poder cachearlos.

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app/models/metadata_file.rb

@@ -13,6 +13,11 @@ class MetadataFile < MetadataTemplate
     value == default_value
   end
 
+  # No hay valores sugeridos para archivos subidos.
+  #
+  # XXX: Esto ayuda a deserializar en {Site#everything_of}
+  def values; end
+
   def validate
     super