Add hardened headers to user-uploaded files (#25756)

dist/nginx.conf

@@ -109,6 +109,8 @@ server {
   location ~ ^/system/ {
     add_header Cache-Control "public, max-age=2419200, immutable";
     add_header Strict-Transport-Security "max-age=63072000; includeSubDomains";
+    add_header X-Content-Type-Options nosniff;
+    add_header Content-Security-Policy "default-src 'none'; form-action 'none'";
     try_files $uri =404;
   }