It might be becuase webwork's FilterDispatcher is not being configured. Since WebWork 2.2.2, ServletDispatcher is being deprecated in favour of FilterDispatcher. FilterDispatcher could be configured as follows

<filter>
        <filter-name>webwork</filter-name>
        <filter-class>com.opensymphony.webwork.dispatcher.FilterDispatcher</filter-class>
  </filter>

  <filter-mapping>
        <filter-name>webwork</filter-name>
        <url-pattern>/*</url-pattern>
  </filter-mapping>

It by default provides all static resources having uri of '/webwork/...'