building with Struts-2.1.3-SNAPSHOT and xworks-2.1.2-SNAPSHOT the XML extenson and XHTML extension seem to work OK both in the showcase and my app, but the .json extension seems to be triggering a File Download action...even in the showcase???
fwiw, JSON serialized data is a key feature of our new architecture...several weeks back this feature worked well and our gov't sponsors were really impressed.
