This is for line breaks handling, ASCII is used to convert line breaks to match the destination operating system.
Basically use ASCII for plain text files (.txt, .php, .js, etc),and use Binary for any other files, or just use the AUTO setting in the FTP client.
More on this in here: https://serverfault.com/questions/110699…
Mirror: https://web.archive.org/web/20230521084…