I don't really see "SH" being used instead of "sh". JPG and JPEG get the uppercase treatment because it is actually an initialism (Joint Photographic Experts Group) unlike "raw".

Some are more used than others, and indeed, JPEG is an initialism. My point is the uppercase treatment doesn't depend on initialism, i.e. RAW doesn't have to stand for something to be capitalized because uppercasing file extensions is just a thing that happens.