After you install Dropbox for Gmail, a new button will appear with a special symbol that you will see every time you start writing a new message. If you click on this icon, the tray that you have activated on your device will automatically open. Even if you haven't linked any accounts, you can still do it directly from Gmail without closing the mail client.
With this new button, attaching files will be much easier. Once you have inserted all the elements that you want to send, you can double check them with the preview option to make sure you're sending the correct files. The best part of this tool lies mainly in its interface, which is exactly the same as Dropbox, so it's much faster to find the folders in which you have stored the files.