Using Classic Outlook on Windows for personal email, calendar, and contact management
I found a solution in one instance.
The end user had two instances of Outlook installed. The default mail app was one version of Outlook, and the one she was trying to drag the attachment out of was another. I changed he default mail app to her preferred Outlook; the one she was dragging the attachment out of. Then she reproduced her process and she was able to drag the attachment as expected.