Major Description
MultiPartRequestWrapper (or StrutsRequestWrapper) is created in Dispatcher.wrapRequest but, there, it's not updated in ServletActionContext.
Probably the update is done later because FileUploadInterceptor.intercept has access to the correct object; but using only StrutsPrepareFilter (instead of an execute one) the method ServletActionContext.getRequest() never return the MultiPartRequestWrapper (or StrutsRequestWrapper).