gistfile1.php

<?php

function featured_image_in_feed( $content ) {
	global $post;
	if( is_feed() ) {
		if( has_post_thumbnail( $post->ID ) ){
			$output = get_the_post_thumbnail( $post->ID, 'large', array( 'style' => 'float:right; margin: 10px;' ) );
			$content = $output . $content;
		}
		if( has_post_format( 'gallery' ) ) {
			$output = '<p>This post contains a gallery. <a href="' . get_permalink( $post->ID ) . '">View the gallery</a> on the website.</p>';
			$content = $output . $content;
		}
		if( has_post_format( 'video' ) ) {
			$output = '<p>This post contains a video. <a href="' . get_permalink( $post->ID ) . '">View the video</a> on the website.</p>';
			$content = $output . $content;
		}
		if( has_post_format( 'audio' ) ) {
			$output = '<p>This post contains some audio. <a href="' . get_permalink( $post->ID ) . '">Listen to the audio</a> on the website.</p>';
			$content = $output . $content;
		}
	}
	return $content;
}
add_filter( 'the_content', 'featured_image_in_feed' );