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; }
}