jp2 - JPEG 2000 files
JPEG 2000 is not yet widely used, but it compresses better than JPEG. At high compression rates the image gets blurry instead of the boxy artifacts with JPEG.
With macOS it has become just as easy to write code for. It may therefore be embedded in files I have not discovered yet. Searching for JPEG 2000 is bundled with search for JPEG.
Safari, Preview and Mail all support JPEG 2000. Photoshop needs a plugin for it. The images below demonstrate the current browser support.
Wikipedia about JPEG 2000 and more references.
JPEG and JPEG 2000 - comparedI compressed a 6MB TIFF image 100 times to make the artifacts visible with JPEG and with JPEG 2000. The html is plain to show what works where.
1 - JPEGScaled down and JPEG compressed to fit in 64 kB