If you want to preserve group of overwritten files, you need to disable transfer resume support.

I've been looking through the documentation, and I was unable to find out if there was a way to have WinSCP automatically set the group name (or gid) of all files I upload. It does preserve the group name if I overwrite files on the server, but by default new files I upload are part of the 'users' group (my user's primary group) but I'd rather they were part of a different group. If this is possible I'd appreciate if somebody could tell me how to set it up. Thanks in advance, and great software by the way!