In Autopilot, you need to hyperlink a file hosted on a service of your choosing (e.g. Dropbox) rather than attaching it to the email like you would in an ordinary email client like Gmail.
To do so, highlight the text you want to link the file to, or click on the image you want to link the file to:
Then paste the file URL:
Click "Ok" and then publish the email and Journey to make your change live.