The "resume" feature is unsafe and the uploaded file is corrupt.
If a file upload is interrupted twice, on the second "resume", the upload will finish instantly and apparently successfully - even when not all of the remaining bytes have been sent. The result is a corrupt file.
In contract, WinSCP 3.5.6 "resume" works perfectly with any number of interruptions.
Additional info follows,
Session protocol = SSH-2
SSH implementation = 188.8.131.52 SSH Tectia Server
Encryption algorithm = aes
Compression = No
File transfer protocol = SFTP-3
The server supports these SFTP extensions: