The violation is trying to tell that the "standard" content type for javascript is application/javascript. unfortunatley the default configuration today returns application/x-javascript.
If you were interested into changing it you need to be careful in updating two pieces of configuration, the httpCompression section and the staticContent section.
staticContent is the one that dictates which content type is sent back, but it is important to update the httpCompression to make sure that compression continues to work for static and dynamic scripts.