There is only one file type that uses the .NBT extension:
Minecraft Named Binary Tag File
Data file format used by Minecraft, a open-ended 3D world building and exploration game; saves data in the Named Binary Tag (NBT) format, which is a simple binary format developed to store large amounts of binary data; used for storing world, or level, information.
The NBT format can be read by software APIs such as JNBT (The Java NBT Library).
NOTE: Files that use the Named Binary Tag format more often use a different file extension, such as .DAT, .MCLEVEL, .MINE, or .MCR.
Updated: June 4, 2018