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When using ASCII mode, files are actually changed, depending on the server. ASCII mode is used to translate ASCII text files between the server and client. For example when uploading to a UNIX server [CR][LF] (carriage return line feed) is converted to just [LF]. And when downloading [LF] is converted to [CR][LF].
This is used to make sure text files are correct for any environment.
When doing this files are actually changed, and the size and content of files are changed. If you’re uploading images or some other binary files it is quite likely that the files will be uploaded or downloaded incorrectly.