manifest.webmanifest or manifest.json #689

I notice the w3c spec mentions manifest.webmanifest yet I see Google and Firefox and other browsers say to use manifest.json

Simple question: Should we use both or just pick one? ~As I can't find the answer in the spec. Plus the spec doesn't mention manifest.json either which seems strange.

(I been using the .json and seen no problems for several years, just wondered if I should add both).

Thanks.