I'm not sure if this should be a new file type. I can see that's it's useful to have an option to store all views in a single file or separately, but does this need to be a file type?

The reason I'm thinking this is to keep it easy to render with and without stereo. EXR multilayer is the file format where you get all the data and can use that for compositing later. With this multiview file format you get in the situation where depending on if you are using stereo you need to use a different file format.

I suggest to make it a file saving boolean option, not a new file format.