A script for uploading image files within WordPress using Redactor.js This script will ensure that images uploaded via Redactor are added to the media library in WordPress.

wp-redactor-image-upload.php

<?php

// Make sure required $_FILES values are set before trying to load WordPress
if ( isset( $_FILES['file'] ) ) {

	// Load WordPress
	require( strstr( dirname( __FILE__ ), 'wp-content', true ) . 'wp-load.php' );

	// Load additional required files since we did a short init

	require( ABSPATH . 'wp-admin/includes/media.php' ); // Required to use the media_handle_upload() function
	require( ABSPATH . 'wp-admin/includes/image.php' ); // Required to use the wp_read_image_metadata() function

	// Define WP_CONTENT_URL to avoid PHP notice
	if ( ! defined( 'WP_CONTENT_URL' ) ) {
		define( 'WP_CONTENT_URL', get_option( 'siteurl' ) . '/wp-content' );
	}

	/**
	 * Class Redactor_Image_Upload_For_WordPress
	 */
	class Redactor_Image_Upload_For_WordPress {

		/**
		 * Handles the upload if a file is available and is of the correct mime type.  On success, a JSON object is
		 * returned to Redactor containing the image URL.
		 */
		public function __construct ( $file_handle ) {
			$url = $this->upload_image( $file_handle );
			if ( $url ) {
				wp_send_json( array( 'filelink' => $url ) );
			}
		}

		/**
		 * Upload an image
		 *
		 * @param array $file_handle
		 * @return string The image URL
		 */
		public function upload_image ( $file_handle ) {
			$upload = media_handle_upload(
				$file_handle,
				0,
				array(),
				array(
					'test_form' => false,
					'mimes'     => $this->get_image_mime_types(),
				)
			);
			$attachment_id = is_wp_error( $upload ) ? 0 : (int) $upload;
			$url = $attachment_id ? wp_get_attachment_url( $attachment_id ) : '';
			return $url;
		}

		/**
		 * Get a list of allowable image mime types.
		 * NOTE: The normal WordPress mime type filter is applied here, so use the 'upload_mimes' filter to customize.
		 *
		 * @return array
		 */
		public function get_image_mime_types () {
			$all_mime_types = get_allowed_mime_types();
			$mime_type_matches = wp_match_mime_types( 'image', $all_mime_types );
			$image_mime_types = array_intersect( $all_mime_types, (array) array_shift( $mime_type_matches ) );
			return $image_mime_types;
		}

	}

	// Let our class do all the work
	new Redactor_Image_Upload_For_WordPress( 'file' );

}