How to force file download in the browser, nginx server

Question

I currently have 2 image locations and they may the formats (jpg,jpeg,png,gif)

i.domain.com/simage.jpg thumbnail
i.domain.com/image.jpg high quality
i.domain.com/o/image.jpg full resolution

Does anyone know how I can force image files in /o/ to download rather than render in the web browser?

Here's my conf file: http://pastebin.com/dP8kZMzx

#setup subdomain i.domain.com
server {
    server_name i.domain.com;
    access_log /var/log/nginx/i..com.access.log;
    error_log /var/log/nginx/i.domain.com.error.log;

    root /var/www/domain.com/test1/images;
    index index.php index.html index.htm;

    #error_page 403 = /notfound.jpg;
    #error_page 500 = /notfound.jpg;

    location / {            
        #change this to a 404 img file .jpg
        try_files $uri $uri/ /notfound.jpg;

        rewrite  "/s([A-Za-z0-9.]+)?" /small/$1 break;
        rewrite  "/o/([A-Za-z0-9.]+)?" /orig/$1 break;
        rewrite  "/([A-Za-z0-9.]+)?" /medium/$1 break;
    }

}

Answer 1

You just need to return HTTP header Content-disposition:

location ~* /orig/(.+\.jpg)$ {
    add_header Content-disposition "attachment; filename=$1";
}