There is only one file type that uses the .MCA extension:
Minecraft "Anvil" Map File
World format used by Minecraft, a game that allows open-ended construction of worlds; replaces the older .MCR (Minecraft Region) format and includes enhancements such as an increased build height (256 up from 128), support for more block types, and more efficient file compression; also enables greater modding capabilities for the game community.
NOTE: The MCA Minecraft map format is referred to as "Anvil," an arbitrary name chosen by the Minecraft developers.
Updated: October 10, 2013