Today, Google announced an important improvement: "Offline Gmail" users are now able to send messages including attachments when they are offline. The upcoming Mailplane 2.1.1 version will allow attachment drag and drop while you are offline.
In case you can't wait and like to test it, here's the Mailplane 2.1.1 preview version.
Like to setup "Offline Gmail"? Check out the Gmail offline 'how-to' article.