Synhronize resume download not working.

Hi!
I launch winscp from windows scheduler, with following options C:\Program Files\WinSCP\WinSCP.com /script=D:\ftptest\ftp.txt.
In ftp.txt set:
open upload
synchronize local D:\ftptest / -criteria=size -resumesupport=on
exit

So, if i manually interrupt file downloading and start scheduler task again, files that were downloaded partially not resumed, but begin to be downloaded again.
Whats not what I want, any help?

And one more question, when files are not fully downloaded, they must have .filepart extension?
What part also not working for me :(

This works with SFTP protocol only. What protocol are you using?