Convert Cropper JS BASE64 image to upload to WordPress media

Question

I am using the Cropper JS to crop an image that I am adding into a WordPress plugin I am developing.

All is fine with the cropping tool and it is sending the image as a BASE64 string via AJAX to my PHP script that I want to upload to WordPress.

I have the correct code to upload a normal image to WordPress, but the image that I pass through for that to work is a normal HTML file upload field.

I am looking for a way of either converting the BASE64 string into a format needed to work with the WordPress upload, or change the WordPress upload functionality to use the BASE64 string instead. I am not sure which is easiest / best - any ideas?

Below is the PHP code that works when passing through an image via an upload field:

<?php

add_action('wp_ajax_crop_image', 'crop_image_callback');
add_action('wp_ajax_nopriv_crop_image', 'crop_image_callback');

function crop_image_callback()
{
    check_ajax_referer('crop_image', 'security');
    $arr_img_ext = array('image/png', 'image/jpeg', 'image/jpg', 'image/gif');
    if (in_array($_FILES['file']['type'], $arr_img_ext)) {
            $upload = wp_upload_bits($_FILES["file"]["name"], null, file_get_contents($_FILES["file"]["tmp_name"]));
            if (!$upload['error']) {
            $post_id = $_POST['post_id'];
            $filename = $upload['file'];
            $wp_filetype = wp_check_filetype($filename, null);
            $attachment = array(
                    'post_mime_type' => $wp_filetype['type'],
                    'post_title' => sanitize_file_name($filename),
                    'post_content' => '',
                    'post_status' => 'inherit'
            );
            $attachment_id = wp_insert_attachment($attachment, $filename, $post_id);
            if (!is_wp_error($attachment_id)) {
                    require_once(ABSPATH . 'wp-admin/includes/image.php');
                    $attachment_data = wp_generate_attachment_metadata($attachment_id, $filename);
                    wp_update_attachment_metadata($attachment_id, $attachment_data);
                    update_post_meta($post_id, 'plain_image', $attachment_id);
            }
            }
            echo $upload['url'];
    }
    wp_die();
}

?>

The BASE64 image string of the cropped image is being passed via POST, as such $_POST['image'];

I can show the jQuery/AJAX if necessary, but that part is working correctly and sending over the string. It is just the PHP code that I want to update to be able to upload that BASE64 string to the WordPress media.