It happens when you need to convert a raw file from Canon, Nikon or Sony DSLR. Usually you will need to have a software for this. Such a software is a raw converter. If you don't have such a program installed on the computer you can't even see the file contents.
There are few services out there that solves this problem. Everything you need is just drop your file right into your browser and that's it. It will first show the file content and then you may convert your file into one of widely used formats like JPG or PNG.
The most convenient and fast service is a Raw.pics.io. It allows to convert nef to jpg, cr2 to jpg, arw to jpg and many others for Canon, Nikon, Sony and other cameras.