Is there a working example of developing an SFTP client that uses a private key file for authentication to send/receive files?
Hi,
If you mean upload/download via SFTP, then the current implementation of TWebUpdate and TWebCopy does not support private key files. However, a quick research shows it should be possible to add support for it but we'll need to allocate time for this.
Great. How much time do you think will be needed?
The implementation itself looks to be straightforward but we need a capable SFTP server to test against as we currently don't have any similar setups available.
Could you provide a test account along with the necessary key files for your server (via an email)? That would significantly reduce the time we need for this.
Unfortunately I don't have an SFTP server. 
I want to develop a client to connect to one.
Hi. Any update on this?
The responsible developer is out of office, and will look into this matter as soon as possible upon return.
Hi,
Unfortunately we could not setup a server that would be suitable for testing, we currently don't have the resources to do so.
Given you want to develop a client to connect to such server, are you sure you cannot provide a server we can test against so that we know the implementation is correct?
Hi. We connect to a live SFTP server. We can't share our credentials for you to test. You can send us a beta version to test ourselves.