We have two media types (GenericMedia & Publication) which inherit media data and have a MediaDescriptor to associate the pdf,doc,docx extensions to both. When a user uploads a file via the asset manager its defaulting to publication but I want it to default to generic media

Ive used this blog post as the basis and would appear to do what I want but in my solution base.GetFirstMatching returns null. Any ideas why?


Snippet

 [ServiceConfiguration(typeof(ContentMediaResolver)) ]
 public class DefaultContentMediaResolver : ContentMediaResolver
 {
 /// <summary>
 /// see http://getadigital.com/no/blogg/resolve-default-media-types-in-episerver/
 /// </summary>
 public override Type GetFirstMatching(string extension)
 {
 var selectedAsDefault = base.GetFirstMatching(extension); //always null?
 
 // For all types inheriting MediaData, I want to use class GenericMedia as default not publication
 if (typeof(MediaData).IsAssignableFrom(selectedAsDefault))
 return typeof(GenericMedia);
 
 return selectedAsDefault;
 }
 }