For some messages with attachments the attachment icon (paper clip) shows in the message list, for some it does not until that message is selected. I'm not sure why some do and some don't, but it happens whether or not they are threaded, and whether or not they are plain, html or multi-part. Sorry, can't find any consistent way to determine which ones do or don't show up. But I never noticed this before, so I think this must be due to a relatively recent revision. I'm currently on 830974. cheers M.
*** This bug has been marked as a duplicate of 154241 ***