How can I upload a local file using fetch ?

## Summary

I want to upload a local file
`const response = await fetch(`${dir}/env.json`);. `
In version electron 11, this worked.
Now I am getting this output in the console

` [webpack-dev-middleware] wait until bundle finished: /index.html
Rewriting GET /Users/denis/env.json to /index.html
[18016:0213/133706.916013:ERROR:CONSOLE(160)] "Electron sandbox_bundle.js script failed to run", source: node:electron/js2c/sandbox_bundle (160)
[18016:0213/133706.916042:ERROR:CONSOLE(160)] "TypeError: object null is not iterable (cannot read property Symbol(Symbol.iterator))", source: node:electron/js2c/sandbox_bundle (160)
`
What setting do I need to change to make this work?
Thanks