FWIW, the "javax.mvc.engine.defaultViewFileExtension" feature would be roughly equivalent to the Spring MVC "suffix" element in the following snippet from WEB-INF/applicantContext.xml:
<bean class="org.springframework.web.servlet.view.InternalResourceViewResolver" id="viewResolver">
<property name="contentType" value="text/html;charset=UTF-8" />
<property name="prefix" value="/WEB-INF/views/" />
<property name="suffix" value=".jspx" />
<property name="viewClass" value="org.springframework.web.servlet.view.JstlView" />
</bean>
Neil