New Post: Cannot save Attachements

Can I ask you for help about this issue with attachment !
Is there any in line solution, I mean without changing the source code ? I currently don't know how to compile the source...if you can give some guides...

                            fl_at.Download();
                            try
                            {
                                fl_at.Save(folderpath); // gives me an  The input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding characters,             or an illegal character among the padding characters.
                            }
                            catch (Exception ex)
                            {
                                MessageBox.Show("Error : " + ex.Message);
                            }

I am connecting to the server, reading all msgs, but in current case I am trying to download a .pdf attachment and I get this error!
C# VS Express 2013-Desktop/ WIN10,64BIT