Confirmed it does work properly if W3TC Browser Cache setting is unchecked: Prevent caching of objects after settings change
May also be the matching routine used by Easy Fancybox. See line ~108+ of easy-fancybox.php
Probably it does not match the filetype if the media query string is appended to the .jpg etc. Need to allow for the w3tc media query string, which is now in the form ?60eab3 -- e.g. file src is
blah.png?60eab3