Added json mimetype Mapping to web.config.template to support things like manifest.json in certain browsers (e.g. IE).

src/Umbraco.Web.UI/web.Template.config

@@ -213,7 +213,9 @@
       <remove fileExtension=".less" />
       <mimeMap fileExtension=".less" mimeType="text/css" />
       <remove fileExtension=".mp4" />
-      <mimeMap fileExtension=".mp4" mimeType="video/mp4" />			
+      <mimeMap fileExtension=".mp4" mimeType="video/mp4" />
+      <remove fileExtension=".json" />
+      <mimeMap fileExtension=".json" mimeType="application/json" />			
     </staticContent>
 
     <!-- Ensure the powered by header is not returned -->