Simple use the Attach file button to add attachments to your email. Each time you attach a file, it will begin uploading in the background. You can immediately click Attach file again to attach another file while a previous one is still uploading, they'll be queued.
If you have some large attachments to add to your email, it's best to do it when you start composing your email. That way they can upload in the background while you continue to type and compose your email, and hopefully will all be completed by the time your down, so sending will then be nearly instant.
You can add files from your File Storage area to your messages as attachments. Click on the 'File Space' button, which will take you to the File Storage screen. Here you can select the checkbox(es) of the files you want to attach, and click the 'Attach' button. The selected files will appear in an attachment list at the top of the screen. You can add as many attachments as you like (up to a maximum of 7M). When you are finished adding attachments, click on the 'Done' button to return to the Compose screen.