MJ2 (also known as Motion JPEG 2000 or Motion JPEG2000) is a video format defined by Part 3 of the JPEG 2000 standard. MJ2 supports audio as well.
According to the specification, MJ2 is one of several specifications with common formatting, including JPEG 2000, MPEG-4, and QuickTime, sharing a common definition for the structure of a file (a sequence of objects, called boxes for JPEG 2000 and atoms for MP4 and QuickTime), and a common definition of the general structure of an object. Codestreams in Motion JPEG 2000 files are JP2 codestreams, as defined for JPEG 2000.
The registered MIME type is video/mj2. At least one document says it is video/mjp2 instead.
The preferred filename extension is .mj2. The MIME type registration lists .mjp2 as an alternate extension.
MJ2 files start with bytes
00 00 00 0c 6a 50 20 20 0d 0a 87 0a ?? ?? ?? ?? 66 74 79 70 6d 6a 70 32.