You need the MediaService to save the item:

var mediaService = Services.MediaService;
mediaItem.SetValue("umbracoFile", "/media/1059/uploader-test.jpg");
mediaService.Save(mediaItem);

Yup - that did the trick... - how could I miss that... ;-)

Hm - since upgrade to ver. 7.3.0/7.3.1 it seems like the mediaService is not acting correctly... :-|

@Robin If you can show more of your code (preferably in text, not in a screenshot) we can help you better.

Absolutly - this is a little script I'm experimenting with, a custom mediaType to take screenshots with - worked like a charm in ver. 7.2.8:

using Newtonsoft.Json.Linq;
using System;
using System.Drawing;
using System.IO;
using System.Linq;
using System.Text.RegularExpressions;
using System.Threading;
using umbraco.cms.businesslogic.web;
using Umbraco.Core;
using Umbraco.Core.Events;
using Umbraco.Core.Models;
using Umbraco.Core.Services;

namespace ScreenDumbDataType.App_Start
{
public class ScreenShot : ApplicationEventHandler
{
    protected override void ApplicationStarted(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext)
    {
        MediaService.Saved += MediaServiceSaved;
    }

    void MediaServiceSaved(IMediaService sender, SaveEventArgs<IMedia> e)
    {
        foreach (var mediaItem in e.SavedEntities)
        {
            if (mediaItem.ContentType.Alias == "Screencapture")
            {
                int imgId = mediaItem.Id;
                string Name = mediaItem.Name;
                string imgName = Name.ToLower()
                    .Replace(" ", "-")
                    .Replace(",", "")
                    .Replace(".", "-")
                    .Replace(":", "")
                    .Replace("æ", "ae")
                    .Replace("ø", "oe")
                    .Replace("å", "aa")
                    .Replace("ä", "ae")
                    .Replace("ö", "oe")
                    .Replace("ü", "ue")
                    .Replace("ß", "ss")
                    .Replace("Ä", "ae")
                    .Replace("Ö", "oe").Trim();
                string imgUrl = mediaItem.GetValue<string>("targetUrl").Trim(); //This Property is crucial for this MediaType

                Thread thread = new Thread(delegate()
                    {
                        using (System.Windows.Forms.WebBrowser browser = new System.Windows.Forms.WebBrowser())
                        {
                            browser.ScrollBarsEnabled = false;
                            browser.AllowNavigation = true;
                            browser.Navigate(imgUrl);
                            //browser.Width = 1920;   //Modify theese as you whant
                            //browser.Height = 1080;   //Modify theese as you whant
                            browser.Width = System.Windows.Forms.Screen.PrimaryScreen.Bounds.Width;
                            browser.Height = System.Windows.Forms.Screen.PrimaryScreen.Bounds.Height;
                            browser.Name = imgName;
                            browser.Tag = Convert.ToInt32(imgId); //Placeholder to pass on the NodeId
                            browser.DocumentCompleted += new System.Windows.Forms.WebBrowserDocumentCompletedEventHandler(DocumentCompleted);
                            while (browser.ReadyState != System.Windows.Forms.WebBrowserReadyState.Complete)
                            {
                                System.Windows.Forms.Application.DoEvents();
                            }
                        }
                    });
                thread.SetApartmentState(ApartmentState.STA);
                thread.Start();
                thread.Join();
            }
        }
    }


    private void DocumentCompleted(object sender, System.Windows.Forms.WebBrowserDocumentCompletedEventArgs e)
    {
        System.Windows.Forms.WebBrowser browser = sender as System.Windows.Forms.WebBrowser;
        using (Bitmap bitmap = new Bitmap(browser.Width, browser.Height))
        {
            browser.DrawToBitmap(bitmap, new Rectangle(0, 0, browser.Width, browser.Height));
            using (MemoryStream stream = new MemoryStream())
            {
                int mediaId = Convert.ToInt32(browser.Tag);
                var mediaService = ApplicationContext.Current.Services.MediaService;
                var mediaItem = mediaService.GetById(mediaId);

                string propertySrc = mediaItem.GetValue<string>("umbracoFile");

                dynamic jsonValue = JObject.Parse(propertySrc);
                string propertyValue = jsonValue.src.Value;

                string root = System.Web.Hosting.HostingEnvironment.MapPath("/media");
                string createImg = browser.Name + ".jpg";

                if (propertyValue  == String.Empty)
                {
                    string newFolder = "";
                    foreach (string folder in Directory.GetDirectories(root).Where(file => Regex.IsMatch(Path.GetFileName(file), "^[0-9]+")).OrderByDescending(x => x).Take(1))
                    {
                        string folderId = Path.GetFileName(folder);
                        int number = int.Parse(folderId);
                        newFolder = (number + 1).ToString();
                    }
                    string createFolder = ((newFolder == String.Empty) ? "1001" : newFolder);
                    string createPath = root + "\\" + createFolder;

                    Directory.CreateDirectory(createPath);
                    bitmap.Save(createPath + "\\" + createImg, System.Drawing.Imaging.ImageFormat.Jpeg);
                    mediaItem.SetValue("umbracoFile", "/media/" + createFolder + "/" + createImg);
                    mediaService.Save(mediaItem);
                }
            }
        }
    }
}
}

Hmmm.. from the code I can only conclude that mediaItem is actually null, have you checked that?

is working on a reply...