There is only one file type that uses the .NKC extension:
Kontakt Library Data File
File used by Kontakt audio sampling software; used along with a corresponding .NKX file, which contains the sample and patch information; uses the same filename prefix as the NKX file but is much smaller in size; used as part of the sample library.
|Native Instruments Kontakt||Commercial|
Updated: December 7, 2012