$ajax file upload not working

Question

I have below code which is submit form using Jquery AJAX with image upload. When ajax call server file I cannot acces $_FILES variable.

function submitForm()
        {  
            var data = $("#ff").serialize();

            $.ajax({

                type : 'POST',
                url  : 'register_process.php',
                data : data,
                contentType: false, 
                cache: false,                   
                processData:false, 
                beforeSend: function()
                { 
                    $("#error").fadeOut();
                    $("#register_btn").html('  sending ...');
                },
                success :  function(response)
                {      
                    if(response=="ok"){

                        //$("#register_form_succ").fadeIn();
                        $("#register_form_succ").html('Registration successful <a href="index.php">Click here</a> to login');
                    }
                    else{

                        $("#error").fadeIn(1000, function(){      
                            $("#error").html(' '+response+' !');
                        });
                    }
                }
            });
            return false;
        }

and HTML below

<form method="POST" action="register.php" enctype="multipart/form-data" id="ff" name="reg">
                <div>
                    <b> I am new user !</b><br/><br/>
                </div>
                <label>
                    <span>Full Name *:</span>
                    <input type="text" placeholder="Please enter Full Name" name="name" id="name" autofocus>
                </label>
                <label>
                    <span>Username *:</span>    
                    <input type="text" placeholder="Please enter Username" name="username" id="username">
                </label>
                <label>
                    <span>Password *:</span>    
                    <input type="password" placeholder="Please enter Password" name="password" id="password">
                </label>
                <label>
                    <span>Email *:</span>    
                    <input type="email" placeholder="Please enter Email" name="email" id="email">
                </label>
                <label>
                    <span>Photo *:</span>    
                    <input type="file" name="photo" id="photo">
                </label>

                <input type="submit" class="sendButton" name="register_btn" name="register_btn" value="Save"/>
                <div>I already Registered.<a href="index.php"> Login here</a></div>                    
            </form>

But in register_process.php i cant get $_FILES variable. it is blank