Yes you are right in your assumption
I am using winscp version 5.11.1 (build 7725) to upload ~15GB of file in background transfer mode. Upload was interrupted in the middle and my PC was restarted leaving .filepart file onto the destination which is of 2.6GB.Now when I started a fresh winscp session to attempt the transfer again without deleting the .filepart at destination, transfer looks like it picked up from where it was interrupted before. It is still using same .filepart file without changing its size. Am I right in assuming that transfer is resumed from the last failed point or there will be an inconsistency in the final file if I keep going like this?