Browser fails to load a rebuilt web app correctly (MIME TYPE '' ERROR)

When building my responsive web app with Iris, I observe a weird behavior. The app build works fine across multiple builds. Then, when selecting clean build, the browser fails when trying to load the web app.

I see the following:

Rebuilding App does not help, nor does restarting Iris or republishing the app help in any way.

What's the reason for this?

Notice to self: The appid is somehow stored in browser cookies and/or local storage. After clearing those, the rebuilt app loads well again. Good to know!