Safari has a file type "Safari download" and appends '.download' to pending/partial downloads. If there is already a pending download w/ same filename, it appends '-X.download' where X is the number of pending downloads with that name.
Orion should do the same. I think it makes sense for it to be a separate file type '.oriondownload', this way we know it's from Orion. If it is just '.download' then it's confusing bc it will say "Safari download" next to it (Finder appears to use the file extension and not some file header to determine file type).