Fixes: #U4-919, we will just use real classes for extension methods and all custom file systems like the MediaFileSystem must inherit from

FileSystemWrapper

src/umbraco.editorControls/tinymce/tinyMCEImageHelper.cs

@@ -136,7 +136,7 @@ namespace umbraco.editorControls.tinymce
 
         private static string doResize(Hashtable attributes, out int finalWidth, out int finalHeight)
         {
-            var fs = FileSystemProviderManager.Current.GetFileSystemProvider<IMediaFileSystem>();
+            var fs = FileSystemProviderManager.Current.GetFileSystemProvider<MediaFileSystem>();
 
             string resizeDim = helper.FindAttribute(attributes, "width") + "," +
                                helper.FindAttribute(attributes, "height");