I would like to change the following REST extension from older version to the new API model in version 7.

[RestExtension("Helper")]
public class Helper
{
    [RestExtensionMethod(returnXml=false)]
    public static string MediaPathById(string id) {
        return new Media(int.Parse(id)).getProperty("umbracoFile").Value.ToString();
    } 
}

I have read the new API documentation and cant figure out the easiest approach to convert this, ideally if possible i would like to do a get with querystring.

public class MediaPathGetter : UmbracoApiController
{       
    [System.Web.Http.AcceptVerbs("GET")]
    [System.Web.Http.HttpGet]
    public string MediaPathById() {

     var test =0;
    var rid= System.Web.HttpContext.Current.Request.QueryString["id"];
     if (int.TryParse(rid, out test)) {
        return  ApplicationContext.Current.Services.MediaService.GetById(int.Parse(rid)).GetValue<string>("umbracoFile");
        }
    return "invalid request";
    } 

If I try the above (hacky) code i get invalid keys for the mediaservice.getbyid method when i pass a valid media id.

Would appreciate if someone can shed light.