Added a unit test to confirm any ctor of a custom typed file system

src/Umbraco.Tests/IO/FileSystemProviderManagerTests.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.IO;
 using System.Linq;
 using System.Text;
 using NUnit.Framework;
@@ -11,7 +12,7 @@ namespace Umbraco.Tests.IO
     public class FileSystemProviderManagerTests
     {
         [Test]
-        public void Can_Get_File_System()
+        public void Can_Get_Base_File_System()
         {
             var fs = FileSystemProviderManager.Current.GetFileSystemProvider(FileSystemProvider.Media);
 
@@ -25,5 +26,26 @@ namespace Umbraco.Tests.IO
 
             Assert.NotNull(fs);
         }
-    }
+
+		[Test]
+        public void Exception_Thrown_On_Invalid_Typed_File_System()
+		{
+			Assert.Throws<InvalidOperationException>(() => FileSystemProviderManager.Current.GetFileSystemProvider<InvalidTypedFileSystem>());
+		}
+
+		#region
+
+	    /// <summary>
+	    /// Used in unit tests, for a typed file system we need to inherit from FileSystemWrapper and they MUST have a ctor
+	    /// that only accepts a base IFileSystem object
+	    /// </summary>
+	    internal class InvalidTypedFileSystem : FileSystemWrapper
+	    {
+		    public InvalidTypedFileSystem(IFileSystem wrapped, string invalidParam) : base(wrapped)
+		    {
+		    }
+	    }
+
+	    #endregion
+	}
 }