I observed following difference in behaviour between two apps, hope it will help to track the problem down.
When I use d-n-d with TotalCMD, once I drag the files and move the mouse cursor over the Outlook mail window, cursor changes to arrow with small empty rectangle. Then I drop the files and they appear as attachments.
When I do the same from WinSCP window, cursor changes to arrow with small rectangle and plus sign. Then I drop the files and nothing happens
That plus sign is the difference between two apps. It is the same difference as when you drag files in Explorer from one window to other, and you hold down the Ctrl key (plus sign occurs) and do not hold the Ctrl key (no plus sign).
Before I hit the Submit button, I simulated once more, and noticed that "plus-sign-cursor behaviour" is different when the mail is set to HTML or plain text. The result (nothing happens when I release the mouse button) is the same. Just with mail set to plain-text, plus-sign appears when mouse cursor is over the window, and when set to HTML, plus sign does not appear.
Versions and other information for bugreport:
WinSCP 5.11.2, build 7781 (but the behaviour is the same for several versions back)
transfer protocol SFTP (but it probably does not matter, since I am talking about Local window)
using GUI, Commander variant