I was trying to put the thumbnails for my custom post types in their respective feed using the
the_excerpt_rss filters. The problem was when I reached for the feed using the url
http://example.com/my_post_type/feed/ nothing happened. The feed was there but with no thumbnails.
Then I found that I can also use the url
http://example.com/feed/?post_type=my_post_type and guess what? The thumbnails were there too!
Now I wonder what could be different about the mentioned urls? I can only guess the filters don't apply for the first.