Source: http://feeds.gawker.com/~r/gizmodo/full/~3/nJtORN50T_w/offline-gmail-gets-attachment-support
According to Google, one of the most requested features for offline Gmail users was the ability to include attachments in emails. Well, that problem has been solved.
Starting today, users can attach all types of files—except images embedded in the body of the email. These messages now go through the outbox when you're online or offline, allowing Gmail to capture the attachment either way. [Gmail Blog via TechCrunch]