it was `zmodem`:

https://b0b.com/ZMODEM.TXT

I was pretty young so I barely knew how to operate the telnet terminal - we used one called White Knight. I recall it had a white background by default (hence the name, I suppose) and a green and yellow pie chart download progress indicator.

Not sure if the protocol was available or easily enabled at the time or via those means. I surely would have used it! Buy my brother was the one who set up the application in the first place.