AJAX PHP上传文件问题

I am uploading multiple files using add more files in ajax and save it in database in php. But it insert only last file in database not multiple file in database.

HTML code

<form id="registration_form" method="post" enctype="multipart/form-data">
   <input type="file"   id="file" name="file[]">
   <input type="button" id="add_more" class="upload" value="Add More Files"/>
</form>

AJAX code

$(document).ready(function() {
    //  To add new input file field dynamically, on click of "Add More Files" button below function will be executed.
    $('#add_more').click(function() {
    $(this).before($("
    <div id='filediv'/>
    ").append($("<input name='file[]' type='file' id='file'>"
    )));
    $.ajax({
    url:"update.php",
    type:"post",
    data: new FormData(this),
    contentType:false,
    cache:false,
    processData:false,
    success:function(data)
    {
    alert(data);
    },
    failure:function(errormsg)
    {
    alert(errormsg);
    }
    });
});`

PHP code

 <!------- Including PHP Script here ------>
 <?php if (isset($_POST['submit'])) {
   $j = 0;     // Variable for indexing uploaded image.
   $target_path = "upload/";     // Declaring Path for uploaded images.
   for ($i = 0; $i < count($_FILES['file']['name']); $i++) {
   // Loop to get individual element from the array
   $validextensions = array("jpeg", "jpg", "png");      // Extensions which are allowed.
   $ext = explode('.', basename($_FILES['file']['name'][$i]));   // Explode file name from dot(.)
   $file_extension = end($ext); // Store extensions in the variable.
   $target_path = $target_path  . $_FILES['file']['name'][$i];  
   //echo $target_path; die;   // Set the target path with a new name of image.
   $j = $j + 1;      // Increment the number of uploaded images according to the files in array.
   if (($_FILES["file"]["size"][$i] < 800000)     // Approx. 800kb files can be uploaded.
   && in_array($file_extension, $validextensions)) {
   if (move_uploaded_file($_FILES['file']['tmp_name'][$i], $target_path)) {
   // If file moved to uploads folder.
   echo $j. ').<span id="noerror">Image uploaded successfully!.</span><br/><br/>';
   } else {     //  If File Was Not Moved.
   echo $j. ').<span id="error">please try again!.</span><br/><br/>';
   }
   } else {     //   If File Size And File Type Was Incorrect.
   echo $j. ').<span id="error">***Invalid file Size or Type***</span><br/><br/>';
   }
   }
   } ?>

You need to append all the files to form data. Try this

var data = new FormData();

$.each($("input[type='file']")[0].files, function(i, file) {
    data.append('file', file);
});

Pass this data variable to ajax data.