Steps to reproduce:

1. using Windows 10 File Explorer, open the folder c:\users*user_name*\AppData\Local\Temp\moz_mapi\
2. copy a PDF file in the above folder (for example, a file called copy.pdf)
3. right click copy.pdf, select "Send to"; the compose window opens with the PDF already attached. At this point, an additional copy of copy.pdf (later called tmp.pdf) has been created in the folder "moz_mapi"; Please note that it is tmp.pdf that has been attached, not copy.pdf.
4. In Thunderbird select "Compose" to open the second compose window
5. Drag the attachment from the first window to the second compose window
6. Close the first compose window and wait; within 10s you will see that the tmp.pdf is deleted

Actual results:

It is impossible to save the draft or send the email because the attachment file refers to a location that is currently unavailable (tmp.pdf has been deleted).

Expected results:

Keeping the same logic, when the attachment is dragged from one compose window to another, an additional temporary copy should be created; each window should keep its own temporary copy so as to properly handle deletion in case of window closure.
Changing logic, it would be sufficient not to create temporary copies of the file and always attach the original file. This ensures that the sent file is always the most up-to-date.