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.
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.