Featured Image in Feed.php

<?php

class DX_RSS_Feed {
	public function __construct() {
		add_filter( 'the_excerpt_rss', array( $this, 'featured_image_in_rss_feed' ) );
		add_filter( 'the_content_feed', array( $this, 'featured_image_in_rss_feed' ) );
	}

	/**
	 * Adds the post's featured image to the RSS feed
	 *
	 * @param $content
	 *
	 * @return mixed|string
	 */
	function featured_image_in_rss_feed( $content ) {
		global $post;
		if ( has_post_thumbnail( $post->ID ) ) {
			$content = '<div>' . get_the_post_thumbnail( $post->ID, 'medium', array( 'style' => 'margin-bottom: 15px;' ) ) . '</div>' . $content;
		}

		return $content;
	}
}