It has been observed with FileZilla Server that it does not acknowledge STOR command after otherwise successful upload of large file. Hence it makes WinSCP think that upload has failed and restarts.
FileZilla client seems to succeed to work. But this is probably only because it seems to simply log "Timeout detected" log entry, without any retry attempt (which it does not support).
Also email to Paul DePesa on Sat, 26 Apr 2008 22:03:07 +0200