How behave getContentType() and why it returns null?

I retrieve the content type with HttpServletResponse#getContentType() when filtering the request inside my ServletFilter but the result is always a null value. Not sure this is a bug since I'm not really sure this is the best way to do. However, in my tests only undertow returns null (not tomcat and jetty).

Can you explain when and why getContentType() returns null with undertow ?

Thanks