This code should work. However, haven't tried it on files.

string mappedUrl;

if (PermanentLinkMapStore.TryToMapped(linkItem.Href, out mappedUrl))
{
    var url = new UrlBuilder(mappedUrl);

    var contentReference = PermanentLinkUtility.GetContentReference(url);
}

But I guess there is new and better approach, maybe with UrlResolver...

Hi Tobias,

The way of Johan is very nice but I think you can base on LinkItem to get it easier:

var mappedHref = linkItem.GetMappedHref();
var contentReference = EPiServer.Web.PermanentLinkUtility.GetContentReference(new UrlBuilder(mappedHref));

// Ha Bui