The FTP service reveals the Operating System type via the 'SYST' command.
This is historically used to determine how to handle file types as many OSes format data differently.
# telnet ftp.microsoft.com 21
Connected to ftp.microsoft.com.
Escape character is '^]'.
220 Microsoft FTP Service
331 Anonymous access allowed, send identity (e-mail name) as password.
230-Welcome to FTP.MICROSOFT.COM. Also visit http://www.microsoft.com/downloads.
230 User logged in.
221 Thank you for using Microsoft products.
Connection closed by foreign host.