Sorry, there is a bug, but not the one I thought it was. Calling 'by hand' the REST interface does show the attachment ID. My issue is with a character in the filename breaking rt_client parsing, I think.
My attachment file name has spaces, commas and dots. I am quite sure that the problem is somewhere here: m=v.match(/(\d+):\s+(.*?)\s+\((.*?)\s+\/\s+(.*?)\)/) I will report back later. Thierry