The MIME type detection on our hosting is just fine, and if you try to open the file manually, you’ll see that the server responds with the right Content-Type header.
However, you’ll notice it doesn’t work outside of your browser. Specifically, because of this security system:
Unfortunately, the consequence of this is that it’s not possible to host a PWA with us.
Small side note, without considering the above, your manifest.json is invalid. You have a comment in your file for the theme_color, however JSON does not support comments. Some parsers will handle Javascript style comments in JSON like you’d expect in Javascript, but it’s still technically invalid JSOn.