From my experience with a couple of family members as Attaches it implies a role, not just on the staff. Air Attache during WW2 from UK to a Commonwealth country and Cultural Attache in 1990s. ETA one was in the Diplomatic Corps and the other had a long military career before and during the war in the UK but was originally from the Commonwealth country to which he was sent.
'An attaché is normally an official, who serves either as a diplomat or as a member of the support staff, under the authority of an ambassador or other head of a diplomatic mission, mostly in intergovernmental organizations or international non-governmental organisations or agencies.'
As I understand it General O'Connor's group was something like a quasi govt or rep in exile.
So an Attache de Servis would have held an important role as an advisor to the General. He may have had experience in the Diplomatic Corps or in protocol or admin matters.