There is only one file type that uses the .XCPLUGIN extension:
Plugin bundle used by Apple Xcode, an iOS and OS X development IDE; stores data referenced by Xcode to extend the application's capabilities, such as colored text or custom compilers; similar to a .BUNDLE file.
XCPLUGIN files should be installed in the ~/Library/Application Support/Developer/Shared/Xcode/Plug-ins directory. If the pathway does not exist, you can create it. Restart Xcode after installation and the plugins will be functional.
Updated: November 26, 2014