Openssh sftp-server: .filepart support?

ProFTPD can support SFTP via the mod_sftp module, and can do just what you describe with its HiddenStores directive; this functionality is supported for both FTP and SFTP uploads.

Full disclosure: I'm the author of ProFTPD and mod_sftp.

The OpenSSH server does not support this.

WinSCP SFTP client can do this from a client side, if that helps.

See https://winscp.net/eng/docs/resume#automatic

By default is uses the .filepart suffix for files over 100 KB only, but you can configure it to use it for any file.

See https://winscp.net/eng/docs/ui_pref_resume

(I'm the author of WinSCP)

Use rsync (over ssh). It's designed to not destroy existing files if copies fail half way through.

If you are able to write Perl, you could write your customized SFTP server on top of Net::SFTP::Server/Net::SFTP::Server::FS.

Update: see sftp-server-atomic-put.pl