In this discussion on the Commons village pump, I got pretty solid support for a proposal to enable uploads of DNG raw image files to Commons:
This is a royalty-free open specification format that fulfills a useful role that existing permitted formats do not, in that it allows other users to create new versions of a photo. See the thread above for details.
All I need from the devs is to enact this by adding 'dng' to $wgFileExtensions for Commons. I would take care of everything else: adding templates to link DNG files with their JPEG versions, adding project pages to explain them, how to use them, and link software support for them. There's no need for thumbnail support for these files, nor would such support make a lot of sense.
For those interested in adding support for a new file type, please read: https://www.mediawiki.org/wiki/Manual:Adding_support_for_new_filetypes