Net::FTP supports binary and text transfer modes only but some systems support another modes. For example, EBCDIC mode ("TYPE E") in IBM mainframes.
Custom transfer modes are needed to be supported by Net::FTP.
What do you expect for custom transfer modes?
Data types' may be more appropriate than transfer modes' according to RFC959.
I don't have IBM mainframes, so I'm not sure what is needed.
I guess it's not enough to provide a way to send "TYPE E".
With EBCDIC type, is used to denote end-of-line.
Should Net::FTP convert from EBCDIC code to ASCII code?
Ruby doesn't support EBCDIC, so it's pain to directly handle EBCDIC characters.