How to implement a MultiFileLoader?

Hello,

I'm missing support for MultiDataObject - current file type tutorials 
don't seem to cover this.

I've got 2 XML files (extensions "layoutml" and "eoml") and an 
additional XML file (with extension "eoml") in a folder (which has a 
fixed name).

I'd like to get all of these as one MultiDataObject (making either the 
layoutml or the outer "eoml" file the primary file - the eoml has some 
metadata, but there exist different eoml's for different datatypes).

I'd also like to only see the primary file, while the other files and 
the extra folder should be invisible to the user.

Kind regards
Peter