Suggested file transfer protocols :
Xmodem - works with telnet
Xmodem/1K - works with telnet
Xmodem/1K-g - works with telnet
Ymodem - works with telnet
Ymodem-g - works with telnet
Zmodem - telnet can be iffy, but is the fastest protocol over dial-up
SeaLink - probably obsolete, unknown performance under telnet
Punter - required for a lot of C= terminal programs, unknown performance under telnet, there is a variety that can transfer multiple files called Multi-Punter
WXmodem - Windowed Xmodem, probably obsolete, unknown performance under telnet
Kermit - highly reliable under telnet & there is a C128 implementation