How to pass an ajax input value using FormData?

I am trying to load images using ajax and php and I managed to make it work at a certain level successfully, but I cannot pass my input value (ID) to my php file.

Here's my scenario.

THE FORM

<form enctype="multipart/form-data" id="myform">
        <input type="file" name="images[]" multiple id="image"/>
</form>

      

        
        
        
      

    

Button

<button type="button" id="uploadimages" name="thing_id" 
value="<?php echo $row['thing_id']; ?>" class="btn btn-primary">Save changes
</button>

      

        
        
        
      

    

AJAX

    $("#uploadimages").click(function () {
        var form = new FormData($('#myform')[0]);

        // Make the ajax call
        $.ajax({
            url: 'uploadimages.php',
            type: 'POST',
            xhr: function () {
                var myXhr = $.ajaxSettings.xhr();
                if (myXhr.upload) {
                    myXhr.upload.addEventListener('progress', progress, false);
                }
                return myXhr;
            },
            //add beforesend handler to validate or something
            //beforeSend: functionname,
            success: function (res) {
                $('#content_here_please').html(res);
                /**/
            },
            //add error handler for when a error occurs if you want!
            //error: errorfunction,
            data: form,
            cache: false,
            contentType: false,
            processData: false
        });
    });

      

        
        
        
      

    

PHP

$get_id = $_POST['']; // This is where I stuck.

      

        
        
        
      

    

How do I pass a variable $row['thing_id']

to $get_id

? I can pass it using another GET ajax call on success, but then I lost the image_id value (I am using foreach since I can upload multiple files) so I want to process it in the same php file.

I didn't include my upload script because it works unless I try to do something with thing_id.