How to upload multiple images through ajax jquery

HTML

<div class="col-md-6" align="right">
    <label>Select Multiple Files</label>
</div>
<div class="col-md-6">
    <input type="file" name="files" id="files" multiple />
</div>
<div style="clear:both"></div>
<br />
<br />
<div id="uploaded_images"></div>

JavaScript

$('#files').change(function(){
   var files = $('#files')[0].files;
   var error = '';
   var form_data = new FormData();

   for(var count = 0; count<files.length; count++)
   {
      var name = files[count].name;
      var extension = name.split('.').pop().toLowerCase();

      if(jQuery.inArray(extension, ['gif','png','jpg','jpeg']) == -1)
      {
          error += "Invalid " + count + " Image File"
      }
      else
     {
        form_data.append("files[]", files[count]);
     }
   }
   if(error == '')
   {
       $.ajax({
          url:"url",
          method:"POST",
          data:form_data,
          contentType:false,
          cache:false,
          processData:false,
          beforeSend:function()
         {
             $('#uploaded_images').html("<label class='text-success'>Uploading...</label>");
         },
         success:function(data)
         {
             $('#uploaded_images').html(data);
             $('#files').val('');
         }
     })
  }
  else
  {
      alert(error);
  }
});

Not same as your question but you can try like this.

Here is your working code. There were several problem with your code

1. Incorrect brace closing in ajax call.
2. Your name field in form data was invalid
3. You were requesting form_data as index in the $_FILES array
4. No use of number variable

index.php

<input id="avatar" type="file" name="avatar[]" multiple="multiple" 
    />
 <button id="upload" value="Upload" type="button">upload</button> 

<div class="preview">
</div>

<div class="return_php"></div>

<script   src="https://code.jquery.com/jquery-3.1.0.min.js" ></script>
    <script>
    $(document).ready(function(){
        var form_data = new FormData(); 

        /* WHEN YOU UPLOAD ONE OR MULTIPLE FILES */
        $(document).on('change','#avatar',function(){
            $('.preview').html("");
            len_files = $("#avatar").prop("files").length;
            for (var i = 0; i < len_files; i++) {
                var file_data = $("#avatar").prop("files")[i];
                form_data.append("avatar[]", file_data);
                var construc = '<img width="200px" height="200px" src="' +  window.URL.createObjectURL(file_data) + '" alt="'  +  file_data.name  + '" />';
                $('.preview').append(construc); 
            }
        }); 

        /* WHEN YOU CLICK ON THE IMG IN ORDER TO DELETE IT */
        $(document).on('click','img',function(){
            var filename = $(this).attr('alt');
            var newfilename = filename.replace(/\./gi, "_");
            form_data.delete($(this).attr('alt'));
            $(this).remove();
        });

        /* UPLOAD CLICK */
        $(document).on("click", "#upload", function() {
            $.ajax({
                url: "upload.php",
                dataType: 'image/png',
                cache: false,
                contentType: false,
                processData: false,
                data: form_data, // Setting the data attribute of ajax with form_data
                type: 'post',
                success: function(data) {
                    //console.log("data")'  
                }
            });
        });
    });
</script>

upload.php

<?php
 //upload.php  
 $output = '';  
 if(is_array($_FILES) && !empty($_FILES['avatar']))  
 {  
      foreach($_FILES['avatar']['name'] as $key => $filename)  
     {  
          $file_name = explode(".", $filename);  
           $allowed_extension = array("jpg", "jpeg", "png", "gif");  
           if(in_array($file_name[1], $allowed_extension))  
           {  
                $new_name = rand() . '.'. $file_name[1];  
                $sourcePath = $_FILES["avatar"]["tmp_name"][$key];  
                $targetPath = "uploads/".$new_name;  
                move_uploaded_file($sourcePath, $targetPath);  
           } 
      }  
     $images = glob("uploads/*.*");  
      foreach($images as $image)  
     {  
           $output .= '<div class="col-md-1" align="center" ><img src="' .  $image .'" width="100px" height="100px" style="margin-top:15px; padding:8px; border:1px solid #ccc;" /></div>';  
      }  
      echo $output;
 }
?>