Hey, I got it fixed.
Look at this open issue for the Go language: mime: incorrect mime-type for .js files on windows 10 (text/plain instead of application/javascript) · Issue #32350 · golang/go · GitHub
On the issue I found a link to this project: GitHub - zekroTJA/go-win-mime-fix: Little tool to fix golangs mime.TypeByExtension issue with js files on windows plattform
I executed that program and it fixed the issue.
Thanks for your help!