es-module-shims | new ES modules features on top of the basic modules support | User Interface library

I'm having a JS issue with my first Rails app, which I suspect is related to my using import maps instead of Webpack. I've searched and searched but haven't found any discussions of this.

It's a Rails 6 app with some JS via Stimulus, which I installed by adding importmap-rails and then stimulus-rails. It works fine locally, but in production on Heroku the JS doesn't work and I see errors like this in the browser console: Uncaught (in promise) Error: Unknown Content-Type "text/html; charset=utf-8" doFetch https://plainreading.herokuapp.com/assets/es-module-shims-424762490b07cc7f9a37745e8b53356bca11997a8e313f47d780e29009df7e8e.js:580

I'm wondering if it's because I removed Webpack from my app, using How to completely remove webpack and all its dependencies from Rails App. I removed it because I was getting Webpack-related build errors in Heroku, and it's my understanding that I don't need Webpack if I'm using import maps.

A while ago I fixed a similar issue in a static site on Netlify by including this in its netlify.toml config:

I've got a Rails 6.1 app which has been upgraded from Rails 4 -> 5 -> 6, so it's mature.

It still uses asset pipeline and with the future of Rails 7.0 I'm going to keep it that way. I wanted to start using stimulus + importmaps, so I've followed along with the install.

After installing importmaps and stimulus through hotwire I'm getting the following errors in Firefox 91 when loading my app;