think this should work.

            SimpleQuery<File> query = new SimpleQuery<File>(@"
                SELECT COUNT(*) TotalFiles, f.AuthorName 
                FROM File f 
                WHERE f.Game.UrlName = :game AND f.Category.Name = :category 
                GROUP BY f.AuthorName");

this should def work.

            SimpleQuery<File> query = new SimpleQuery<File>(@"
                SELECT COUNT(*) as TotalFiles, f.AuthorName 
                FROM File f 
                WHERE f.Game.UrlName = :game AND f.Category.Name = :category 
                GROUP BY f.AuthorName");