Messages cannot accept file transfers
Communicating just fine between two computers on Messages, but one of the computers gets an error message when we try to attach a small picture of any kind. It says that the intended recipient cannot accept file transfers, yet the other user can send them to that user just fine. Both computers in the same room on the same wifi network (doing that to test, not to avoid talking to each other in the same room!)
The computer that is getting the error message can't send to other users either. Same error message, so it's not just one recipient that "cannot accept file transfers", which makes me feel that it's an issue with the sending computer.
iMac, OS X Mountain Lion