IValueConverter Issue

 I am using a FileToImage converter to display a image based on the file extension.

Everything works for normal extensions, but if the filename has a extension that is not in the media folder of the project, then no image is being displayed.  I want to try and assign a generic icon for all file extensions that do not have a extension in the folder.

Here is the converter I am using:

It works if we have a file named file_extension_docx.png in the Media folder. 

How can I display a generic image for the extensions that are not in the folder?

public class ConvertFileImageConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            if (value as string == string.Empty) return null;
            string file_extension = System.IO.Path.GetExtension(value.ToString());
            file_extension = file_extension.Replace(".", "");
            var file = @"/Media/file_extension_" + file_extension + ".png";
            ImageSource src = new BitmapImage(new Uri(file, UriKind.Relative));
            return src;
        }

        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }