This script handles WordPress image uploads when uploading files using Redactor.js. It uses the 'SHORTINIT' parameter to prevent loading plugins and unnecessary files. NOTE: Images uploaded using this script are NOT imported into the WordPress media library.

redactor-image-upload.php

<?php

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

	// We don't need all of WordPress, just the basics
	define( 'SHORTINIT', true );

	// 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-includes/l10n.php' ); // Required to gain access to the __() function
	require( ABSPATH . 'wp-includes/formatting.php' ); // Required to gain access to the untrailingslashit() function
	require( ABSPATH . 'wp-includes/post.php' ); // Required to gain access to the wp_match_mime_types() function
	require( ABSPATH . 'wp-admin/includes/file.php' ); // Required to gain access to the wp_handle_upload() 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();
			if ( $url ) {
				wp_send_json( array( 'filelink' => $url ) );
			}
		}

		/**
		 * Upload an image
		 *
		 * @param array $file
		 * @return string The image URL
		 */
		public function upload_image ( $file ) {
			$upload = wp_handle_upload(
				$file,
				array(
					'test_form' => false,
					'mimes'     => $this->get_image_mime_types(),
				)
			);
			return ! empty( $upload['url'] ) ? $upload['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' );

}