There is only one file type that uses the .MJ2 extension:
Motion JPEG 2000 Video Clip
Video format defined as Motion JPEG 2000 (Part 3); contains a motion sequence of JPEG 2000 images; does not involve inter-frame coding, but instead encodes each frame independently using JPEG 2000 compression.
MJ2 files may be produced by digital cameras that capture video. They are also used in medical and satellite imagery.
Updated: July 2, 2009