The title just about says it all.
I found a similar question on the fossil-users mailing list, and Richard Hipp responded here saying just to add the "b" capability for the user. As the user in question had "s", I thought this was unnecessary, but it would have been rude not to try, which I did. I still can't see any way to add an attachment.
I'm using chiselapp and I'm sure that I'm missing something obvious. If someone could steer me to what that something is, I'd be very grateful.
You can attach files to a ticket that's already been created (there's an "Attach" link at the top-left of the page when you view the ticket) but I don't think you can do it (create a ticket with an attachment) in a single action.