Community commons is the way to go here indeed. But, why do you need the fileGUID in the first place? In most cases it makes more sense to use the fileID instead of the guids. Note that prior Mendix 4 guids are not constant over time.
The only way to do this is Java. Pass the filedocument as an object. Code is something like.
// BEGIN USER CODE myobject.setMyGuidString(myobject.getId().toString()); myobject.Commit(); // END USER CODE
I found a solution, the mendix community commons has a java action that retrieves the GUID.