hmm... thanks dzurn... "metadatas" is definitely out of the options! ;-)
But dzurn, are you sure that there's "metadatum" word in English? I couldn't find it in dictionary (only on wiktionary).
Anyway, I am not sure about using metadatum. If we look at the "file"/"item"/"object" element, it actually contains metadata and not metadatum. Maybe a group of metadatum for each "item" element. So it's actually metadata. The whole XML file would be a group of metadata (which again can be called as metadata, I guess).
Looking at the options again I think "datafile" name with "ref" attribute or maybe "path" attribute sounds correct. The only concern would be - "Is it quite distinguishable from the File group?"... I think yes.
But as I said before, I am quite comfortable with any name... :-)