I know that this issue is explained in Javadoc - Context#getRequestParameter().
Actually, irrelevant Controls to Form interfere in the processing of
the multipart data.
I want to expand the chance that FileField configurations is applied.
Especially, when it wants to limit the data size by using the function of
Commons FileUpload, I feel the necessity.
ActionLink and ActionButton are sure to function only by HTTP GET Method
if my understanding is correct.
The attached patch is the one to fix to check whether the request is
a Content-type "multipart/form-data" POST request, before
Context#getRequestParameter is called in Action(Link|Button)#onProcess.