Implemented a real .Net configuration section for umbracoSettings.config + unit tests for every property. NOTE: This isn't referenced in the codebase yet, going to start setting default values for most of the properties so that we can remove them from the config file for shipping (ship minimal config) then reference these settings in the codebase.

src/Umbraco.Core/Configuration/UmbracoSettings/AppCodeFileExtensionsCollection.cs

@@ -0,0 +1,31 @@
+using System.Collections.Generic;
+using System.Configuration;
+
+namespace Umbraco.Core.Configuration.UmbracoSettings
+{
+    internal class AppCodeFileExtensionsCollection : ConfigurationElementCollection, IEnumerable<FileExtensionElement>
+    {
+        protected override ConfigurationElement CreateNewElement()
+        {
+            return new FileExtensionElement();
+        }
+
+        protected override object GetElementKey(ConfigurationElement element)
+        {
+            return ((FileExtensionElement)element).Value;
+        }
+
+        IEnumerator<FileExtensionElement> IEnumerable<FileExtensionElement>.GetEnumerator()
+        {
+            for (var i = 0; i < Count; i++)
+            {
+                yield return BaseGet(i) as FileExtensionElement;
+            }
+        }
+
+        System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
+        {
+            return GetEnumerator();
+        }
+    }
+}