Using WinSCP 5.15.1, Thunderbird 60.7.0, Windows 10 1809, but it's been occurring for quite a period of time before. Connection is over SCP, and using the Commander GUI.
I say sometimes, because sometimes it does work as expected - but for somefiles it often exhibits this behaviour. I will try and get a script to show this.
WinSCP remote target has an empty directory. x.pdf is an attachment for a mail held in Thunderbird. Drag and drop to WinSCP folder gives x-1.pdf. delete the file in the WinSCP target directory, and repeat. file is now x-2.pdf.
If at this point you also drag the file into the local side of WinSCP (again to a folder which has never had this file in) gives x-2.pdf
The contents of the file are always correct.
Dragging some other file types (eg .ZIP) does not cause the -n to be added. I don't know the full list of what does add it, and what doesn't, but .ZIP. .SEF don't.
Drag and drop the file from Thunderbird to Windows downloads gives x.pdf. Drag and drop from Windows to WinSCP gives x.pdf, so it's something to do with the Thunderbird-WinSCP interface.