"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," mentions Gmail's blog.
Offline Gmail is still a Gmail Labs feature and it requires Google Gears. If you use Google Chrome, Gears is already installed. The nice thing about Offline Gmail is that you can read recent conversations, compose messages and view your contacts even when you don't have Internet access.