WIP - bit of a refactor on the IFile stuff, moves all validation to the service level and obsoletes the IFile.IsValid methods. Adds v5 style CSS property parsing, but might integrate that with this zany css parser in the core for 'safer' parsing.

src/Umbraco.Web/Trees/TemplatesTreeController.cs

@@ -129,7 +129,7 @@ namespace Umbraco.Web.Trees
         {
             //TODO: Rebuild the language editor in angular, then we dont need to have this at all (which is just a path to the legacy editor)
 
-            return template.GetTypeOfRenderingEngine() == RenderingEngine.WebForms
+            return Services.FileService.DetermineTemplateRenderingEngine(template) == RenderingEngine.WebForms
                 ? "/" + queryStrings.GetValue<string>("application") + "/framed/" +
                   Uri.EscapeDataString("/umbraco/settings/editTemplate.aspx?templateID=" + template.Id)
                 : "/" + queryStrings.GetValue<string>("application") + "/framed/" +