So I want to directly append/write things into an existing csv-file under medias only (not first creating one locally etc...), without having to use this method: HttpContext.Current.Server.MapPath(@"~\files\" + media.Name + ".csv");

I've tried this, but of course some errors appeared, as it can't write into Uri paths or something:

var media = Umbraco.TypedMedia(einladung.SelectedCsvMedia);
                if (media != null)
                {

                    var path = Request.Url.GetLeftPart(UriPartial.Authority) + media.Url;
                    var data = Environment.NewLine + "\"" + Request["email"] + "\";\"" + Request["surname"] + "\";\"" + Request["name"] + "\";\""+ Request["inputTextArea"]+"\";\""+registration+"\"";
                    var header = "\"email\";\"firstname\";\"lastname\";\"comment\";\"register\"";
                    IMediaService mediaService = ApplicationContext.Current.Services.MediaService;
                    var mediaFile = ApplicationContext.Current.Services.MediaService.GetById(media.Id);
                    if (media.GetProperty("umbracoBytes") == null || media.GetPropertyValue<int>("umbracoBytes") <= 5)
                    {
                        System.IO.File.WriteAllText(path, header);

                    }
                    System.IO.File.AppendAllText(path, data);

                    mediaService.Save(mediaFile);
                }

Then I tried this, but it isn't a valid virtual path:

 var media = Umbraco.TypedMedia(einladung.SelectedCsvMedia);
                if (media != null)
                {

                    var path = HttpContext.Current.Server.MapPath(Request.Url.GetLeftPart(UriPartial.Authority) + media.Url);
                    var data = Environment.NewLine + "\"" + Request["email"] + "\";\"" + Request["surname"] + "\";\"" + Request["name"] + "\";\""+ Request["inputTextArea"]+"\";\""+registration+"\"";
                    var header = "\"email\";\"firstname\";\"lastname\";\"comment\";\"register\"";
                    IMediaService mediaService = ApplicationContext.Current.Services.MediaService;
                    var mediaFile = ApplicationContext.Current.Services.MediaService.GetById(media.Id);
                    if (media.GetProperty("umbracoBytes") == null || media.GetPropertyValue<int>("umbracoBytes") <= 5)
                    {
                        System.IO.File.WriteAllText(path, header);

                    }
                    System.IO.File.AppendAllText(path, data);

                    mediaService.Save(mediaFile);
                }

Is there another way to write into an existing media file?