On Android I’m using the very useful setting of not downloading stuff when the message is over X kB. This is useful, but actually only when on mobile data.
What I would love to have is to skip that when I’m on wifi: download new messages completely, and download messages that were incompletely downloaded. Technically it’s not so much about wifi but unconstrained connection i.e one that is cheap and one that is expensive; wifi/not wifi is a good proxy for that
(I’m sure this is tracked sowmewhere but I couldn’t find it, soooo 
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