Exception: 'Server cannot set status after HTTP headers have been sent.' after Controller successful return

Question

So there are some Q&A on SO, but noone helped me and i figured out what causes the problem, but don't know what to do.

My ReadFiles Controller:

public FileStreamResult ReadFiles(int id, string fileName)
        {
            //Response.BufferOutput = true;
            string extension = Path.GetExtension(fileName).ToLower();

            try
            {
                using (NpgsqlConnection con = new NpgsqlConnection(_Connection))
                {
                    con.Open();
                    using (NpgsqlCommand cmd = new NpgsqlCommand())
                    {
                        cmd.Connection = con;
                        cmd.CommandText = "...";

                        using (NpgsqlDataReader reader = cmd.ExecuteReader())
                        {
                            byte[] chunk = new byte[10 * 1024];
                            string contentType = String.Empty;

                            if (reader.Read())
                            {

                                if (extension.Length > 0)
                                {
                                    Response.Clear();
                                    Response.Buffer = true;
                                    //Response.BufferOutput = true;

                                    Response.Charset = "";
                                    Response.Cache.SetCacheability(HttpCacheability.NoCache);
                                    Response.BinaryWrite((byte[])reader["f_file"]);
                                }
                                else
                                {
                                    long read;
                                    long offset = 0;
                                    read = reader.GetBytes(0, offset, chunk, 0, chunk.Length);
                                    chunk = new byte[read];

                                    do
                                    {
                                            //write output
                                    }
                                    while (read > 0);
                                }
                            }
                                string responseHeader = HelperClass.GetExtensionString(extension);
                                Response.AddHeader("Content-Type", responseHeader);

                                //download file + opens pdf NOT in browser
                                fileName = fileName.Replace("/", "_");
                                Response.AddHeader("Content-Disposition", "attachment; filename*=UTF-8''" + Uri.EscapeDataString(fileName));
                                contentType = Response.ContentType;
                                //Response.SuppressFormsAuthenticationRedirect = true;
                                //Response.StatusCode = 200;
                                //Response.Flush();
                                //Response.Clear();
                                Response.End();

                                return File(Response.OutputStream, contentType, fileName);
                        }
                    }
                }
            }
            catch (HttpException ex)
            {
                Debug.WriteLine(ex.Message);
            }
            catch (Exception ex)
            {
                ....
            }
            //finally
            //{
            //    Response.Clear();
            //    Response.End();
            //}
            return null
        }

When I am running this, it is ALWAYS going into the Global.asax into the Application_Error:

 protected void Application_Error(object sender, EventArgs e)
    {
        Exception ex = Server.GetLastError();
        Debug.WriteLine(Response.StatusCode);
        Debug.WriteLine(ex.GetType().Name);

        ///...
    }

The exception is Server cannot set status after HTTP headers have been sent.

Weird things for me, the StatusCode in the Application_Error is 200, the Exception is an HttpException (which should be catched in the controller).

So I tried many solutions written on SO (put them into comments), but nothing worked. So I figured out what is my problem. Setting the contentType at the beginning doesn't help. When I am commenting this line:

//contentType = Response.ContentType;

Application_Error isn't touched and it is running well (Ok I get the plain text in the browser and no Download appears). Obviously I need the Respones.ContentType in the File returning. It is only working when the ContentType is an empty string. So return File(Response.OutputStream, "", fileName); is working. How I can solve this problem or where do I have to put the content type, that this is working?

Can it be a Serverside problem?