return content-type response headers with a node app behind nginx

Question

We have an web application build using node js (express js), which is behind nginx.

For a particular API, we want to have the content-type response header as "text/plain". For this following code is there in controller.

res.setHeader('Content-Type', 'text/plain'); res.send(response);

This works when the server is not behind nginx. But when the server is behind nginx, the response headers are still 'application/json'

nginx configuration is pasted below:

#kZyguser www-data;
user root;
worker_processes 4;
pid /var/run/nginx.pid;

events {
        worker_connections 20000;
        # multi_accept on;
}

http {

        ##
        # Basic Settings
        ##

        sendfile on;
        tcp_nopush on;
        tcp_nodelay on;
        keepalive_timeout 65;
        types_hash_max_size 2048;
        # server_tokens off;

        # server_names_hash_bucket_size 64;
        # server_name_in_redirect off;

        include /etc/nginx/mime.types;
        default_type application/octet-stream;

        ##
        # Logging Settings
        ##
        access_log /var/log/nginx/access.log;