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 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;
        }
    }

Snippet

    [ContentType(
        DisplayName = "Publication",
        GUID = "457c2e33-cc01-4653-9958-2726027484dc"]
    [MediaDescriptor(ExtensionString = "pdf,doc,docx")]
    public class Publication : MediaData, ISearchableContent
    {
        public virtual bool ExcludeFromSearch { get; set; }
    }

 Snippet

    [ContentType(
         DisplayName = "GenericMedia",
         GUID = "47DA9013-68A0-4897-BBE7-C83A1F1A9BFD",
         Description = "Used for generic file types such as Word or PDF documents.")]
    [MediaDescriptor(ExtensionString = "pdf,doc,docx")]
    public class GenericMedia : MediaData, ISearchableContent
    {

         public virtual bool ExcludeFromSearch { get; set; }
     }