Then you probably need to install the .NEF codec from Nikon's website so your computer can "decode" the RAW file.Raw Paul.
You could also do what ever processing you want to do with them, convert to jpegs and transfer. If memory serves me the chrome books don't have a large HD. Raw images are going to eat up your gigs in a hurry. Or, are you keeping them on a SD card or thumb drive?
Can't you install an app that can read RAW?
Something like this:
https://chrome.google.com/webstore/detail/polarr-photo-editor-raw/djonnbgfieijldcieafgjcnhmpcfpmgg
Until you find the proper codec program that will accept NEF files on your ChromeBook, I suggest that you shoot NEF and Jpeg, move the NEF files on a disk for future use and just transfer the Jpegs on your ChromeBook. There might come a day when the Book will open NEF files but I would't hold my breath. Processing NEFs takes a lot of computer power.