This request has been added to the tracker:
https://winscp.net/tracker/1651

Thanks Martin.
Could winscp be enhanced to allow copying to a temporary file for all FTP protocols, even if resume support is not supported by the protocol?

Transfer to temporary file name is supported with SFTP protocol only:
https://winscp.net/eng/docs/resume#automatic

winscp.com synchronize resumesupport=on is not storing the file being transferred to a temporary filename first.
Resource monitor shows that winscp is writing to the target filename, not a temporary file.
If I interrupt and resume the synchronisation, it starts the copy from 0%.

Details:
Windows Server 2016
winscp.com v5.13

Connection details, from winscp.ini, (TLS/SSL Explicit encryption):
[Sessions\backup.worryfree.co.nz]
HostName=pmrrep.advantage.co.nz
PortNumber=21
FSProtocol=5
Ftps=3

Command (from PowerShell session):
----------------------------------
winscp.com -command 'open "backup.worryfree.co.nz"' 'synchronize local "U:\backups\ShadowProtect\gls2-check\test" "/gls2/test" -resumesupport=on -criteria=size' -log='resumeSupport.log'

I get the same behaviour with:
winscp.com /script=S:\temp\winscp.script
and S:\temp\winscp.script:
option echo on
open "backup.worryfree.co.nz"
synchronize local "U:\backups\ShadowProtect\gls2-check\test" "/gls2/test" -resumesupport=on -criteria=size