Treat custom file extension as CSharp

Hi, 

I'm trying to open up files of type '.mcsx' as if they were .cs files in Visual Studio 2012 - i.e. with code completion and with rich formatting. I've tried a number of things to no avail:

1) I've tried writing a VSIX extension that registers a custom ContentType:

 static class ContentType
    {
        public const string Name = "MyCScript";

        [Export]
        [Name(Name)]
        [DisplayName("CSharp Script")]
        [BaseDefinition("CSharp")]
        public static ContentTypeDefinition MarkdownModeContentType = null;


        [Export]
        [ContentType("CSharp")]
        [FileExtension(".mcsx")]
        public static FileExtensionToContentTypeDefinition MkdFileExtension = null;

    }

2) I've also tried creating an extension package following this blog post: http://blogs.msdn.com/b/noahric/archive/2010/03/01/new-extension-css-is-less.aspx. I managed to grab the appropriate GUID from the registry and create a VSIX with just a pkgdef file with the following entries (and included it in the vsix):

[$RootKey$\Languages\File Extensions\.mcsx]
@="{A066E284-DCAB-11D2-B551-00C04F68D4DB}"

[$RootKey$\Editors\{A066E284-DCAB-11D2-B551-00C04F68D4DB}\Extensions]
"mcsx"=dword:00000028

In this case the entries appear in the registry, but I'm still not getting CSharp completion for my mcsx files. Can anyone help me?