You don't exclude them by file type. You need to get to Excluded Processes, and then add the various processes for MC:
http://wiki.jriver.com/index.php/Taming_Windows_DefenderOne thing to note, if you are running Windows 7, then Windows Defender is NOT an anti-virus application, only a spyware removal tool. It is confusing because Microsoft changed the names of things when Windows 8 was released. This is explained here:
http://blogs.microsoft.com/cybertrust/2013/11/14/windows-defender-and-microsoft-security-essentials-which-one-do-i-need/If you are running Windows XP, Vista, or Windows 7, then the Microsoft Anti-Virus application is called
Microsoft Security Essentials. It isn't included, but you can download it for free here:
http://windows.microsoft.com/en-us/windows/security-essentials-downloadYou can install a separate program from Microsoft called Windows Defender on Windows 7 and earlier, but it is a different type of tool (which removes spyware). You do not need to exclude Media Center in it if you have it. I would probably recommend against using it if you are on Windows 7, though. If you need to remove Spyware, I'd recommend MalwareBytes discussed above instead. But, please note that Windows Defender for Windows XP, Vista, and 7 is NOT an anti-virus application and won't detect or remove virus infections.
From your screenshot, I think this is what you have. It is hard to tell because your screenshot is oddly cropped and doesn't show the whole window, but I think that's what you have there. You have the wrong thing installed, if it is called Windows Defender and you're really on Windows 7.
If you are running Windows 8 or newer, Microsoft merged the functionality previously in Windows Defender and Microsoft Security Essentials together into one tool called Windows Defender.