Why is ASP.NET MVC asking me to re-authenticate for .PNG files but not .gifs or .jpegs?

Question

What am I missing here??? For some reason, anytime I reference a .png from my application.css file I get prompted for credentials. BUT, I can reference .gifs, .jpegs, etc... from my images directory no problem.

Routes are set up right now like this ...

public static void RegisterRoutesTo(RouteCollection routes) {

        routes.IgnoreRoute("{resource}.axd/{*pathInfo}");
        routes.IgnoreRoute("elmah.axd");
        routes.IgnoreRoute("{*favicon}", new { favicon = @"(.*/)?favicon.ico(/.*)?" });

        routes.MapRoute(
            "Default", "",                   
            new { controller = "Home", action = "Index"}  // Parameter defaults
        );

        routes.MapRoute(
            "Session", "{action}",
            new { controller = "Session" },
            new { action = "(login|logout|register)" }
        );



        routes.MapRoute("CatchAll",  "{*catchall}",
            new { controller = "Error", action = "NotFound" });
    }

In my application.css I have this:

.iconLocationLarge { background-image: url(../images/icon_vcarea_48x48.png) !important;}
.iconVCLarge { background-image: url(../images/tb-btn-sprite.gif) !important;}

The Problem: I get prompted to authenticate again if I use a .png ... BUT not with the .gif. Why???

Answer 1

I am not sure why, but probably you should add ignore routes for all static content regardless.

http://stackoverflow.com/questions/2730335/asp-net-mvc-thinks-my-virtual-directory-is-a-controller/2730472#2730472

Answer 2

IIS can serve content directly without passing the request to ASP.NET. I suspect in your case that IIS is set to serve GIF and JPG but not PNG. See for example http://mvolo.com/blogs/serverside/archive/2006/11/10/Stopping-hot_2D00_linking-with-IIS-and-ASP.NET.aspx for a discussion on the topic. See also http://www.microsoft.com/technet/prodtechnol/WindowsServer2003/Library/IIS/4c840252-fab7-427e-a197-7facb6649106.mspx?mfr=true.

Assuming this is IIS6, open the IIS Manager from Administrative Tools, right click on the local computer node, click on MIME types, check that PNG is registered as "image/png". You can also configure this at the individual web site level.

Make sure there is no other filter that could cause the issue, e.g. the UrlScan ISAPI filter.

Answer 3

Finally solved:

The PNG files I was referencing were encrypted. That is it.

After finally exhausting all other options with IIS and ASP.NET I noticed that each PNG file I was referencing had the attributes "AE" (E= Encrypted). So the solution was to right-click the folder, click "Advanced" button and de-select the "Encryption" option.