Steps to reproduce:
1) Create a file in Windows (eg: TEST.txt).
2) Open WinSCP and connect to the destination server using SFTP.
3) In WinSCP, browse to the directory where the TEST.txt is located.
4) In Windows Explorer, rename the file to "test.txt" (just changing the case, the name itself must remains).
5) In WinSCP, upload the file to the destination server.
6) In the destionation server, the uploaded file will be named "TEST.txt" instead of "test.txt".
If in WinSCP I do a refresh on the source pane between steps 4 and 5, the file will be created with the correct case on destination.
I'm using WinSCP 5.17.6 (build 10516) with Commander interface and Windows 7 Ultimate (Build 7601 ) SP 1.