I using Apple Mail 6.1/Mountain Lion with Google IMAP. When I move a mail message with an attachment (pdf, doc, xls, jpg, etc) from my In Box in Apple Mail to another mailbox the attachment disappears. However, when I look at the same messange in Google the attachment is still attached. This is a recent development. Attachment stay connected on another Mac I use.
iMac, OS X Mountain Lion, MacBook Air, IPad v1, Apple TV v2