How to modify a specific RSS feed, without affecting others?

Hi all

I have a site with 2 separate feeds – one for the main blog, and the other for a custom post type:

Blog Posts feed – [WEBSITE URL]/feed
CPT feed – [WEBSITE URL]/feed/?post_type=podcasts

In order for the podcasts feed to validate in iTunes, I append a direct link to the MP3 file using the following code:

function mtg_podcast_include($content) {// Including the podcast link in the podcast RSS feed
	global $wp_query;
	$postid = $wp_query->post->ID;
	$mtg_podcast_url = get_field('mtg_podcast_file_url');
	if(is_feed()) {
		if($mtg_podcast_url !== '') {
			$content = $content.'<a href="'.$mtg_podcast_url.'" rel="enclosure">Download MP3</a>';
		} else {
			$content = $content;
		}
	}
	return $content;
}
add_filter('the_excerpt_rss', 'mtg_podcast_include');
add_filter('the_content', 'mtg_podcast_include');

The problem is, a download link is also appearing at the end of each post in the blog posts feed. Can anyone tell me how I go about modifying only the podcasts feed?

Any help is appreciated!