The modification of "file-upload" module fixes the problem of course, but I wanted to provide a package "EAR" out of the box without modifying a lot the configuration of JBoss AS.
The problem is that another EAR (from another product, GateIN) rely on the predefined "file-upload" module and as you know two JARs loaded in ClassLoaders (interdependent) makes problem especially for Casting and operations like "Class.isAssignableFrom".