Another update to this - beyond having to include a choice list with media filenames to enforce Central attachment upload currently.
If the question is a select_one/multiple_from_file using a CSV that has image and big-image columns in order to show media against the choices (and these have already been included in an internal choice list to force upload so the files are available), these are ignored and the display in Collect will not show the images, I can only make this work with an internal choice list.
For now I have to move questions with images back to internal choice lists, as well as keeping an unused choice list with media filenames when these are used as dynamic default images looked up from a CSV, but would prefer to avoid both these steps as keeping all these data in CSVs allows for a more modular form that's reusable with only updated CSVs.
(related thread on Enketo not handling image/big image and choice filters)