Using Classic Outlook on Windows for personal email, calendar, and contact management
What I find odd, is if you're using the browser version of outlook, you can reply all, and if you add a recipient, then there is a pop up that says "Do you want to add the attachment for 'new recipient.' ?"
This feature is now live in "New" Outlook desktop version. Just noticed it today!
Just a the new recipient and look for the small message at the bottom of the reply window.