Bug with ajax file upload. What's wrong?

Question

AJAX

Hello!
Very strange behavior of the script bundle below. The bug is that "once again" the handler reports that the action is empty ..
and it either loads or not ..

jQuery(document).on('click', '#media-file-upload', function(event){ 
        
    event.stopPropagation(); event.preventDefault();  			
    if( typeof files == 'undefined' ) return( showResponse("error","Вы не выбрали файл") );			
    var data = new FormData(); var action = 'rotator_upload_media';
    data.append("action", action);
    $.each( files, function( key, value ){ data.append( key, value ); });			
    
    $.ajax({
      url         : ajaxurl,
      type        : 'POST',
      data        : data,
      cache       : false,
      dataType    : 'text',
      processData : false,
      contentType : false,
      success     : function( respond, status, jqXHR ){ 
        console.log(respond);
        //showResponse(status, "Загрузка успешно завершена");
        //$.each( JSON.parse(respond), function( key, value ){ console.log( key+": "+value ); });	
      },
      error: function( jqXHR, status, errorThrown ){	showResponse("error","ОШИБКА AJAX запроса: " + status +" " + jqXHR ); },
    });
  });

if(isset($_SERVER['HTTP_X_REQUESTED_WITH']) && !empty($_SERVER['HTTP_X_REQUESTED_WITH']) && strtolower($_SERVER['HTTP_X_REQUESTED_WITH']) == 'xmlhttprequest') {

    if( isset($_POST['action']) ){

      switch ($_POST['action']) {
        case 'rotator_update_options':	echo json_encode(rotator_update_options($_POST['options'])); break;	
        case 'rotator_upload_media':	json_encode(rotator_upload_media($_FILES)); break;	
      }

    }
    
    elseif( isset($_GET['action']) || !empty($_GET['action']) ){
        
      switch ($_GET['action']) {
        case 'rotator_get_options'	 :  echo json_encode(rotator_get_options()); break;
      }
    }

    else { die("Ошибка выполнения: AСTION пуст или неверен!"); }

  }

unction rotator_upload_media($files){ 
    
    $uploaddir = '../medialib';
        
    if( ! is_dir( $uploaddir ) ) mkdir( $uploaddir, 0777 );

    foreach( $files as $file ){
        $file_name = cyrillic_translit( $file['name'] );

        if( move_uploaded_file( $file['tmp_name'], "$uploaddir/$file_name" ) ){            
            $path = "$uploaddir/$file_name";
            $ThisFileInfo['path'] = $path;
            echo json_encode($ThisFileInfo);
        }
            
    }    
}

Answer 1

maybe HTTP_X_REQUESTED_WITH is not always set on the server, dig in this direction, because if this if does not work, then nothing is displayed and, in a sense, ajax does not receive anything.
or else, ajax itself doesn't submit the form. but if it always sends, but sometimes does not receive a response, then it's in php