//StaticCache .cs
using System;
using System.Web;

namespace StaticContentCacheModule
{
   public class StaticCache : IHttpModule
   {
       public void Init(HttpApplication context)
       {
           context.PreSendRequestHeaders += context_PreSendRequestHeaders;
       }

       static void context_PreSendRequestHeaders(object sender, EventArgs e)
       {
           //capture the current Response 
           var currentResponse = ((HttpApplication)sender).Response;

           if (CacheManager.ShouldCache(currentResponse.ContentType))
           {
               currentResponse.AddHeader("cache-control", "public");
               currentResponse.AddHeader("Expires", DateTime.Now.Add(TimeSpan.FromDays(30)).ToString());
           }
       }

       public void Dispose() { }
   }
}

//CacheManager.cs
using System;

namespace StaticContentCacheModule
{
   class CacheManager
   {
       public static bool ShouldCache(string contentType)
       {
           contentType = contentType.ToLower();
           string[] parts =
               contentType.Split(
                   new[] { ';' },
                   StringSplitOptions.RemoveEmptyEntries
               );

           if (parts.Length > 0)
               contentType = parts[0];

           bool cache = false;

           switch (contentType)
           {
               case "text/css":
                   cache = true; break;
               case "text/javascript":
               case "text/jscript":
                   cache = true; break;
               case "image/jpeg":
                   cache = true; break;
               case "image/gif":
                   cache = true; break;
               case "application/octet-stream":
                   cache = true; break;
               default:
                   {
                       if (contentType.Contains("javascript"))
                           cache = true;
                       if (contentType.Contains("css"))
                           cache = true;
                       if (contentType.Contains("image"))
                           cache = true;
                       if (contentType.Contains("application"))
                           cache = true;
                   }
                   break;
           }

           return cache;
       }
   }
}

<httpModules>
     <add name="StaticContentCacheModule" type="StaticContentCacheModule.StaticCache, StaticContentCacheModule"/>
</httpModules>

<system.webServer>
<modules>
  <add name="StaticContentCacheModule" type="StaticContentCacheModule.StaticCache, StaticContentCacheModule"/>
</modules>