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// show post thumbnails in feeds  
function diw_post_thumbnail_feeds($content) 
{
  global $post;
  if(has_post_thumbnail($post->ID)) 
  {
    $content = '<div>' . get_the_post_thumbnail($post->ID) . '</div>' . $content;
  }
  return $content;
}
add_filter('the_excerpt_rss', 'diw_post_thumbnail_feeds');
add_filter('the_content_feed', 'diw_post_thumbnail_feeds');