Do you do that when they send a message "here's some documents", or would you send a thumb up if the message said "Attached is a proposal for the client, are you OK if we send it out?"
Like, surely, in that case you'd treat a thumb up as a dangerous message to send as an ACK? :-)
As well, do you follow your thumbs up eventually with something else? Because if I read it correctly, farmer had from March to November to follow-up on the thumbs up ... as well, there was previous precedent with that exact farmer, which makes it hard to argue "Oh, those previous times I meant thumbs up as acceptance of contract, but in this case I meant is as just an acknowledgment" :-/