You can't as far as I am aware. You can specify the default location thats within Tools and Options but the attachments will appear on a seperate line and not in the subject line.. Thats a new on me, that you could ever do that..
OnePlus 5T review: Hands-on
Illustrator Andrés Lozano on his improv line work, brazen use of colours & hand sketching
iPhone X review
Comment envoyer gratuitement des gros fichiers ?