One of the most requested features for Offline Gmail has been the ability to include attachments in messages composed while offline. Starting today, attachments work just the way you would expect them to whether you are online or offline (with the exception that when you're offline you won't be able to include inline images). Just add the attachment and send your message.
If you have Offline Gmail enabled, you'll notice that all your mail now goes through the outbox, regardless of whether you're online or offline. This allows Gmail to capture all attachments, even if you suddenly get disconnected from network. If you're online, your mail will quickly be sent along to its destination.