In version 2016R1 we've added a new
CompressionLevel property into
ftp.Settings so that you are able to change the Compression level used in mode Z.
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Please note that the setting will only be effective for uploading files, as the compression level used when downloading is determined by the FTP server.
As to your other question - if you turn on mode Z, and the FTP server does not support it, the Rebex FTP client will automatically fallback to mode S (FtpTransferMode.Stream). We have improved the FTP Mode Z documentation on our features page - thanks! This being said, setting the Zlib mode is safe like this whether the server supports it or not:
Ftp ftp = new Ftp();
ftp.TransferMode = FtpTransferMode.Zlib;