Refused to execute script because strict MIME type checking is enabled

I have setup an NGINX reverse proxy to a web service (www.aaa.com) through another domain name (www.bbb.com) while adding a few additional pages to the latter.

Request come from www.bbb.com (nodejs app) but they need to look like they are coming from www.aaa.com or I will have a CORS (Cross-origin resource sharing) issue. Hence NGINX to tweak the headers.

NGINX config

worker_processes 1;
events {
    worker_connections 1024;
}
http {
    include mime.types;
    include servers/*;
    default_type application/octet-stream;
    sendfile on;
    keepalive_timeout 65;

    server {
        listen 443 ssl;
        server_name www.bbb.com;
        ssl_certificate /etc/pki/nginx/server.crt;
        ssl_certificate_key /etc/pki/nginx/private/server.key;
        ssl_session_cache shared:SSL:1m;
        ssl_session_timeout 5m;
        ssl_ciphers HIGH:!aNULL:!MD5;
        ssl_prefer_server_ciphers on;

        location / {
            proxy_set_header X-Real-IP 127.0.0.1;
            proxy_set_header Host www.aaa.com;
            proxy_set_header Upgrade $http_upgrade;
            proxy_set_header Connection "upgrade";
            proxy_pass https://www.aaa.com;
            proxy_http_version 1.1;
        }
    }
}

Looks like one of the resources doesn't get the correct MIME Type through the reverse proxy https://www.aaa.com/scripts/some.script.api.js

Error

Refused to execute script from 'https://www.bbb.com/scripts/some.script.api.js' because its MIME type ('text/html') is not executable, and strict MIME type checking is enabled.

text/html is incorrect indeed, I would be expecting application/javascript

1. Why is this happening? I figured that MIME Types are just set automatically? Could that be that www.aaa.com has implement some new CORS security rule?

2. Is there a way to tell NGINX to set the correct MIME Type for that particular file? I have tried a few things to no avail.

e.g.

add_header Content-Type application/javascript;

but I may have used wrong. Any pointers?

Similar issue described on another Stackoverflow question