New Post: How can i add the MediaFileSelector widget to a parameter in a Razor function?

I am trying to do something like this:
@functions {

[FunctionParameter(WidgetMarkup = "<f:widgetfunction name='Composite.Widgets.String.VisualXhtmlEditor' xmlns:f='http://www.composite.net/ns/function/1.0'><f:param  name='ClassConfigurationName' value='custom' /></f:widgetfunction>")]
public string Description { get; set; }

}

But with a MediaFileSelector. I am not sure how the namespace is.

There is more documentation for that? I saw this page: http://docs.composite.net/ASP-NET/Razor/Changing-Widgets, but only talks about adding the XHTMLVisualEditor.

Regards.