What is the protocol used to transfer files over the internet?
A modem only transmits the data, the computer uses a protocol to
connect to the ISP. Different types of protocol exist, but I think
you're probably thinking of FTP - File Transfer Protocol.
If the modem is used to connect to an ISP for an internet
connection, than the standard internet protocols and utilities can
be used for file transfer. They include ftp, sftp, http and the
wget command. If the modem is used to establish a serial connection
such as a terminal emulation session, protocols like kermit and
xmodem/ymodem/zmodem should be available that perform errorfree
file transfers.