Gatsby: new /html functionality not clear

So I upgrade and I get this message:

Gatsby is currently using the default template for HTML. You can override
this functionality by creating a React component at "/html.js"

You can see what this default template does by visiting:
https://github.com/gatsbyjs/gatsby/blob/master/lib/isomorphic/html.js

Already I am confused, because I have an html.jsx in my root directory and the previous version of gatsby picked it up fine. Maybe I need to put it somewhere else now with the new version?

So I end trying both /wrappers and /pages, neither of which work either
So then I try changing the extension to .js because I use .jsx, and that doesn't work either

Other than this 1 issue, upgrading to 0.10 was painless, but I need code that's in html.jsx, and it's not clear to me why this behavior would have changed?

These are the placements that I have tried:

/html.js
/wrappers/html.js
/pages/html.js

Is there something obvious I'm missing?