This is a booked guest, @Carmela27? You cannot attach to the usual system messages, but what I have done successfully in the past is attached to the temporary address Airbnb sets up for your booked guests. You find that in the area beside their messages in your inbox. It is a long string of numbers and letters that ends in @guest.airbnb.com.
If this is a prospective guest (not booked yet), you cannot send them attachments.