WordPress: Add featured image to RSS feed for use in MailChimp campaign

functions.php

add_filter('the_excerpt_rss', 'webendev_featured_to_RSS');
add_filter('the_content_feed', 'webendev_featured_to_RSS');
/**
* Add featured image to RSS feed for use in MailChimp campaign
*
*/
function webendev_featured_to_RSS($content) {
	global $post;

	if ( has_post_thumbnail( $post->ID ) ){
		$content = '<a href="' . get_permalink( $thumbnail->ID ) . '" title="' . esc_attr( $thumbnail->post_title ) . '" target="_blank">' . get_the_post_thumbnail( $post->ID, 'thumbnail', array( 'style' => 'float:left; margin: 0px 12px 10px 0;' ) ) . '</a>' . $content;
	}

	return $content;

}