Hi Matthias,

You'll need to do the saving in the ProcessValue method that you can overwrite

Here is the code from the default upload 

public override List<object> ProcessValue(HttpContextBase httpContext)
        {
            List<Object> vals = new List<object>();

            //files
            bool filesaved = false;

            var files = httpContext.Request.Files;
            if (files.Count > 0 && files.AllKeys.Contains(this.AssociatedField.Id.ToString()))
            {
                HttpPostedFileBase file = null;
                file = files[this.AssociatedField.Id.ToString()];
                if (file.ContentLength > 0)
                {
                    string dir = Configuration.Path + "/files/" + Guid.NewGuid().ToString() + "/";
                    if (!System.IO.Directory.Exists(httpContext.Server.MapPath(dir)))
                        System.IO.Directory.CreateDirectory(httpContext.Server.MapPath(dir));
                    file.SaveAs(httpContext.Server.MapPath(dir + file.FileName));
                    vals.Add(dir + file.FileName);
                    filesaved = true;
                }

            }

            if (!filesaved)
            {
                vals.Add(httpContext.Request[this.AssociatedField.Id.ToString() + "_file"] ?? "");
            }

            return vals;
        }