WinSCP supports resuming file transfers with SFTP protocol only. SCP protocol does not allow it.
When transferring file with SFTP protocol,
it is first transferred into a temporary file with
Only after the transfer is completed the extension is removed.
If your transfer is interrupted, a partial file (with
.filepart extensions) remains in the target folder. After you try to transfer the file
a second time, WinSCP looks for the partial file. If it finds a partial file, it offers you chance to resume the transfer
(the confirmation can be disabled).
For performance reasons, WinSCP only supports automatic file transfer resumption for files larger then a given threshold. By default, the threshold is 100 KiB. This can be configured on Background & Resume tab of Preferences dialog. Also resuming text mode transfers is not supported.
There's an alternative manual method to resume transfer. The method is useful for resuming transfers initiated by other clients and also to update log files.
If you attempt to transfer a file that already exists in the target directory and the target file is smaller then the source file, the overwrite confirmation dialog displays a Resume button or an Append button (when appending is supported). After you click Append, WinSCP opens another prompt that lets you append source file or resume your transfer.
If you select to resume transfer, the part of source file equal to size of the target file will be skipped and only the remaining part will be transferred.
Note that you must have overwrite confirmations enabled to use the method.
Manual resuming of text mode transfers with SFTP-4 and newer is not supported.
If you want to resume your transfer, you must use the same source file version as the interrupted transfer. If you do not, your target file will include the source file's previous version and its current version. For automatic resume, if the partial file is bigger than the source file, WinSCP will display error and disallow resume.
If the target file exists and it is not write-able, but the target directory is, the file will may be transferred anyway with automatic resume. The error overwriting the file will occur only after the transfer finishes. It is because the file is first transferred into temporary file.