Re: There is no option to enable that, and, as far as I know, Aquamail does not automatically re-download on WiFi what was only partially downloaded on mobile network.
For IMAP, if a message was synced on mobile data, let's say it was "headers only" and then when the device is connected to WiFi, there is a "preload attachments" or "preload inline images" or a "load message text when checking mail" --
-- then yes, it will fetch what it missed, even in those messages which it fetched earlier on mobile data.
For EWS it's the same way.
This is what (others) users expect.