How to attach a file in the Unibox
Click the "Attach Files" button, it is located next to the "Send" button.
Upload the file from your computer.
Once you've attached your file(s) and composed your email, click the "Send" button to deliver your message along with the attached file(s). The email will be sent once the files get uploaded to the Unibox.
If you go back to open the sent message in the Unibox thread, you will see the attached files at the bottom of the sent email: