Unable to cache images served by Spring MVC

I am trying to serve some assets using a Spring MVC controller. My assets are database managed and thus have to be served this way. The service looks up the metadata of the asset from the database, reads the file from file system and builds the response.

Here is how my controller looks like.

@Controller
@RequestMapping("/assets")
public class AssetController {

    @Autowired
    private AssetService assetService;

    @RequestMapping("/{assetName:.+}")
    public ResponseEntity<byte[]> getAsset(@PathVariable("assetName") String assetName) throws FileNotFoundException, IOException {
        Asset asset = assetService.findByName(assetName);

        HttpHeaders headers = new HttpHeaders();
        headers.setContentType(MediaType.valueOf(asset.getContentType()));
        headers.setCacheControl("max-age=1209600");
        headers.setLastModified(asset.getModifiedOn().getTime()); // always in the past

        return new ResponseEntity<byte[]>(assetService.toBytes(asset), headers, OK);
    }
}

Seems simple and straightforward enough? One would hope to see the browser caching the images. But despite trying all combinations of Cache-Control, Expires, Last-Modified-On and ETag, I have had no success.

Below are the HTTP headers (irrelevant headers removed) spit out during two successive requests.

GET /adarshr-web/assets/Acer.png HTTP/1.1
Host: localhost:8080
Pragma: no-cache
Cache-Control: no-cache

HTTP/1.1 200 OK
Cache-Control: max-age=1209600
Last-Modified: Sun, 21 Jul 2013 11:56:32 GMT
Content-Type: image/png
Date: Tue, 23 Jul 2013 21:22:58 GMT
----------------------------------------------------------

GET /adarshr-web/assets/Acer.png HTTP/1.1
Host: localhost:8080
If-Modified-Since: Sun, 21 Jul 2013 11:56:32 GMT
Cache-Control: max-age=0

HTTP/1.1 200 OK <-- Why not 304 Not Modified?
Cache-Control: max-age=1209600
Last-Modified: Sun, 21 Jul 2013 11:56:32 GMT
Content-Type: image/png
Date: Tue, 23 Jul 2013 21:23:03 GMT

However, when I try the same sequence (Ctrl + F5 for first request and F5 for subsequent ones) on URLs such as

http://www.google.co.uk/images/srpr/logo4w.png (Google's logo)
http://fbstatic-a.akamaihd.net/rsrc.php/v2/yI/r/0PsXdTWc41M.png (Facebook's mobile image)

I see the headers such as these (shown for the Facebook URL) which indicate that the response is being cached by the browser.

GET /rsrc.php/v2/yI/r/0PsXdTWc41M.png HTTP/1.1
Host: fbstatic-a.akamaihd.net
Pragma: no-cache
Cache-Control: no-cache

HTTP/1.1 200 OK
Content-Type: image/png
Last-Modified: Sat, 15 Jun 2013 00:48:42 GMT
Cache-Control: public, max-age=31535893
Expires: Wed, 23 Jul 2014 21:27:47 GMT
Date: Tue, 23 Jul 2013 21:29:34 GMT
----------------------------------------------------------

GET /rsrc.php/v2/yI/r/0PsXdTWc41M.png HTTP/1.1
Host: fbstatic-a.akamaihd.net
If-Modified-Since: Sat, 15 Jun 2013 00:48:42 GMT
Cache-Control: max-age=0

HTTP/1.1 304 Not Modified <-- Note this
Content-Type: image/png
Last-Modified: Sat, 15 Jun 2013 00:48:42 GMT
Cache-Control: public, max-age=31535892
Expires: Wed, 23 Jul 2014 21:27:47 GMT
Date: Tue, 23 Jul 2013 21:29:35 GMT

Notes:

I don't have an <mvc:resources /> section in my Spring config since I am doing exactly the same in my controller. Even adding it doesn't make any difference.
I don't have a org.springframework.web.servlet.mvc.WebContentInterceptor defined in the Spring config again for the reasons above. I have tried adding one with no gain.
I have tried all methods explained in https://developers.google.com/speed/docs/best-practices/caching.
I can replicate this across all browsers.

You'll have to implement the check of the last modified, fortunately Spring makes that pretty easy.

From the Spring Framework Reference

@RequestMapping
public String myHandleMethod(WebRequest webRequest, Model model) {

    long lastModified = // 1. application-specific calculation

    if (request.checkNotModified(lastModified)) {
        // 2. shortcut exit - no further processing necessary
        return null;
     }

    // 3. or otherwise further request processing, actually preparing content
    model.addAttribute(...);
    return "myViewName";
}