There is only one file type that uses the .MCSERVER extension:
Minecraft Server File
Server package created and used by MCServer, an application that enables you to build and customize your own Minecraft server; contains several files, including .JAR, .COMMAND, .TXT, and .YML files, used to configure the server.
NOTE: You can create the MCSERVER file with MCServer Maker and open it with MCServer Runner. Both programs are available in the MCServer download.
Updated: December 1, 2015