That is KDE specific, and has nothing whatsoever to do with the OS.
How does your KDE handle file types when you are logged into a headless server, without any kind of graphics hardware, and your console is a serial port? How does the UNIX kernel handle file types when you're on a headless server with no graphics hardware, and your output is stdout on a teletype terminal or a nullmodem cable on a serial port?
I guess you missed the point of everything I've written so far. Did you read the links I provided?
Using desktop environments. If I click on a text file in KDE, it opens in kate. Same with every other linux desktop I've used and same one mac.
You're yet to say a single thing that doesn't apply equally on windows and unix.