"Or just middling stupid & won't try 'cos it's not .docx"
True but in their defense I would appreciate they are noticing such details and not blindly doing stuff they are unfamiliar with. In todays climate you need that "suspicion" as part of your line of defense against attacks.
However once told that odt is a standard and good format I'm sure there will be some who keep forgetting and those that suddenly think that opening that odt file is what caused their laptop to run slow so they ask for one of the new flashy models the noticed we are just starting to give out.