First, you to not need to have the .filepart extension to be able to resume. Just select Resume command on Overwrite confirmation dialog (FTP protocol) or Append, followed by No on the Append dialog (SFTP protocol):
https://winscp.net/eng/docs/ui_overwrite

To enable the transfer to temporary file, see:
https://winscp.net/eng/docs/ui_pref_resume

Note that this works with SFTP protocol only. With SCP protocol, transfer resuming is not supported at all.

For details see:
https://winscp.net/eng/docs/resume

Dear all,

I am trying to download some large files from a remote server using WINSCP. But the files that are being downloaded donot have a .filepart. So when the download is interrupted, I have to redownload from the beginning and overwrite what have been download last time. How could I switch on the .filepart extension function of WINSCP? Thanks in advance.