Using Classic Outlook on Windows for personal email, calendar, and contact management
There is a way to do this, I haven't gone and read the history here and maybe this isn't a good option for most because of the format of the received emails, here are the steps to include the attachment on a reply email in outlook:
File\Options\Mail\Reply and forward settings\ When replying to a message: Attach original message
another way
Obviously, this is a common issue for many, I often "Reply to All" with an attachment, adding more recipients. Until there is a MS fix, I found the easiest and quickest way to add an attachment, from the original message, right-click and copy the attachment (you can also choose this from the dropdown to the right of the attachment), I paste it in the body of the replay email and add additional recipients needing the attachment. It's not even two seconds of time to do this.
on forwarding its always included with these settings,
File\Options\Mail\Reply and forward settings\when forwarding | included and indent original message