I was recently told that commons might find it useful to upload jp2 files, because some glams really like this format.
It would be very easy to add support to this to MW since image magick supports jp2, and in fact, we already support this in a sense, as PDFs often have embedded jp2 files which we already support decoding.
The only really question here I think, is how closely we have to validate that jp2 files are really jp2. The internet suggests that jpx (aka JPEG2000 part 2) files are sometimes mislabeled as JP2 (aka JPEG2000 part 1) files. JPX has less support than JP2 (although image magick claims to support both, I'm unsure how true that claim is), and there is more uncertainty about the patent situation with JPX. Which naturally leads to the question of if we should allow jpx, and if we don't, do we need to verify that jp2 files are really jp2 and not jpx. If we need to verify really jp2, there's a python script that does that at https://github.com/openpreserve/jpylyzer