There is only one file type that uses the .NXC extension:
Not eXactly C Source Code File
Source code file written in the Not eXactly C (NXC) programming language; may be used to program a LEGO Mindstorms NXT brick; can be opened with Bricx Command Center (BricxCC) to view the code with syntax highlighting; can also be viewed with a basic text editor.
NXC is a high-level language that is similar to C and is built on top of the Next Byte Codes (NBC) compiler.
Updated: September 22, 2011