When I drag & drop a file into the Attachments field nothing happens. Although I'm now using 4.80 this has been happening with 4.73 for a long time.

No problems here with drag & drop. I can drop onto the attachment tab itself or into the white space of an open attachment tab.
Does drag & drop work elsewhere on your PC?

Drag & drop does work with all my other Windows apps.

Is there something in the pmail.ini file or another file that control drag & drop? Alternatively, what files & registry entries need to be deleted to completely remove Pegasus? Maybe that would be a fix.

AFAIK, there is nothing in the Pegasus Mail configuration that should affect drag & drop. Neither would any of it registry entries. Those only keep track of the location of the last run instance and provide the necessary entries so that Pegasus Mail can set as the default email program.

I don't know what could be causing your drag & drop problem. Are you dragging from a file explorer and dropping onto the attachment pane?

I am dragging from File Explorer and dropping into the Attachment pane.

The only other thing that came to mind is a drag & drop failure due to the target file being open but Pegasus Mail pops up an explanatory notice about this. Other than that I am clueless.