mime-types.php

<?php
/**
 * Multiple mime types for WordPress.
 *
 * @wordpress-plugin
 * Plugin Name: Multi-mime.
 * Description: Uploaded images in multiple mime types, requires https://github.com/WordPress/wordpress-develop/pull/2239.
 * Plugin URI:
 * Version:     1.0.0
 * Author:      Adam Silverstein, Google
 * License:     Apache License 2.0
 * License URI: https://www.apache.org/licenses/LICENSE-2.0
 */

/**
 * Add additional mime type meta data when generating image sub sizes.
 */
function filter_image_editor_output_formats( $mime_types ){
	if ( isset( $mime_types['image/jpeg'] ) && ! isset( $mime_types['image/jpeg']['image/webp'] ) ) {
		array_push( $mime_types['image/jpeg'], 'image/webp' );
	}

	return $mime_types;
}
add_filter( 'image_editor_output_formats', 'filter_image_editor_output_formats' );