MIME type ('text/html') is not a supported stylesheet MIME type, and strict MIME checking is enabled. about mailman HOT 3 OPEN

server {

 listen 443 ssl http2;
        server_name XXX.YY;

        ssl_certificate /etc/letsencrypt/live/XXX.YY/fullchain.pem;
        ssl_certificate_key /etc/letsencrypt/live/XXX.YY/privkey.pem;
        ssl_trusted_certificate /etc/letsencrypt/live/XXX.YY/chain.pem;

        include snippets/ssl.conf;
        include snippets/letsencrypt.conf;
        include snippets/php-fpm.conf;

        #return 301 https://XXX.YY$request_uri;
        index index.php index.html index.htm index.nginx-debian.html;
        root /var/www/html;

        add_header X-Content-Type-Options nosniff;
location /mailman {
                proxy_pass       http://127.0.0.1:4000/;
                proxy_set_header Host      $host;
                proxy_set_header X-Real-IP $remote_addr;
#               proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        }
}