Setting header instead of adding header with proxy_pass and proxy_hide_header in Nginx

gistfile1.txt

    location ~ /dl/.*\.(jpg|png)$ {
        proxy_pass http://test.51bmb.com;
        proxy_hide_header Content-Type;
        rewrite ^/(.*)/dl/(.*)$ /$1/$2 break;
        root /mnt/data/bmbtest;
        add_header Content-Type application/octet-stream;
        add_header Content-Disposition "attachment;filename=qrcode.png";
        expires 30d;
    }