Go to the paper clip then click on it.
You should get this once you click it. Once you do that then click on Browse first.
Then chose your file that you want to upload. Once you clicked 'opened' you then go back and click upload (from picture above).
Once your done with that then you can go back to the paperclip and click on the arrow with the file, then you just click on the file name and have it attached to the message box.