Individual email messages contain all data and info about their attachments. However, Mail also recreates the attachments in an associated Attachments folder and uses this as a cache to speed up any subsequent access to an attachment. These cached Attachment folders can consume quite a bit of space (mine total 1.8G) and old ones in particular could be deleted without any repercussions. Personally, I wouldn't bother just to free up disk space but I did find one Mail client which seemed to have got itself confused and ended up with many unnecessary cached folders. Deleting all the Attachment folders was the easiest way to clean things up. The following Terminal commands can be used to report on existing Attachments folders and delete them if required.