$('.submit_add').click(function(e){ 
          e.preventDefault();
          var data = $('#myform').serialize();

          console.log(data); return false;
          $.ajax({ 
              url: '/ajax',
              type: 'POST',
              cache: false, 
              data: data,
              dataType: 'json',
              success: function(data) {
                          if (data.success == true ) {
                            window.location.href  = '/';
                          } else {
                            alert('Error : There is something wrong.');
                          }
                        },
              error: function(jqXHR, textStatus, err){
                   alert('text status '+textStatus+', err '+err);
               }
          })
      }); 

Replace the above code by

$('.submit_add').click(function(e){ 
          e.preventDefault();
          var data = new FormData($("#myform")[0]);


          console.log(data); return false;
          $.ajax({ 
              url: '/ajax',
              type: 'POST',
              cache: false, 
              data: data,processData: false,contentType: false,
              dataType: 'json',
              success: function(data) {
                          if (data.success == true ) {
                            window.location.href  = '/';
                          } else {
                            alert('Error : There is something wrong.');
                          }
                        },
              error: function(jqXHR, textStatus, err){
                   alert('text status '+textStatus+', err '+err);
               }
          })
      });