Wordpress attachment images: generate a grayscale copy of main (full) image referenced as an image_size

grayscale.php

/**
 * Generate a grayscale copy of main (full) image referenced as an image_size
 */

function add_grayscale_image_size( $image_sizes ) {
	$image_sizes[] = 'grayscale';

	return $image_sizes;
}

add_filter( 'intermediate_image_sizes', 'add_grayscale_image_size' );

function grayscale_images_filter( $meta ) {
	$file     = wp_upload_dir();
	$filepath = $file['path'];
	$file     = trailingslashit( $filepath ) . $meta['file'];
	$newfile  = preg_replace( '/(\.(png|gif|jpg|jpeg))/', '-grayscale$1', $file );
	list( $orig_w, $orig_h, $orig_type ) = @getimagesize( $file );

	$image = wp_load_image($file);
	if ( is_resource( $image ) ) {
		imagefilter($image, IMG_FILTER_GRAYSCALE);
	}

	switch ($orig_type) {
		case IMAGETYPE_GIF: imagegif( $image, $newfile ); break;
		case IMAGETYPE_PNG: imagepng( $image, $newfile ); break;
		case IMAGETYPE_JPEG: imagejpeg( $image, $newfile ); break;
	}

	$grayscale = [
		'file'   => str_replace( trailingslashit( $filepath ), "", $newfile ),
		'width'  => $meta['width'],
		'height' => $meta['height'],
	];

	$meta['sizes']['grayscale'] = $grayscale;

	return $meta;
}

add_filter( 'wp_generate_attachment_metadata', 'grayscale_images_filter' );