Saving Blob Object As a File On Server

Using jQuery plugin called cropper, I am able to retrieve the cropped image as blob object. Now I need to save this blob object as a file on my server. The code for that is:

$('#image').cropper('getCroppedCanvas').toBlob(function (blob) {
    var formData = new FormData();

    formData.append('croppedImage', blob);
    formData.append('form_key', window.FORM_KEY);

    $.ajax('/upload.php', {
        method: "POST",
        data: formData,
        processData: false,
        contentType: false,
        success: function (data) {
            console.log(data);
        },
        error: function (data) {
            console.log(data);
        }
    });
});

Here is my upload.php:

<?php
$blob = $_POST['croppedImage'];
//return $blob;
file_put_contents('/media/crop_products/test.png', $blob);
?>

It is not saving it on the server. What I am dong here? This request is made from an admin page in magento.