There is only one file type that uses the .MCO extension:
Live Messenger Winks File
Contains Flash-based animations, called Winks, that can be sent using Windows Live Messenger (formerly MSN Messenger); similar to sending an emoticon, but can be a much larger animation.
Winks may be installed using Messenger Jump! MSN Winks Installer or MSN BackUp.
NOTE: Windows Live Messenger was discontinued in 2013 and has moved to Skype.
|Microsoft Windows Live Messenger|
Updated: October 4, 2015