Add featured image in rss feed in a wordpress site

code.php

<?php

function add_featured_image_in_rss() {

    if ( function_exists( 'get_the_image' ) && ( $featured_image = get_the_image('format=array&echo=0') ) ) {
        $featured_image[0] = $featured_image['url'];
    } elseif ( function_exists( 'has_post_thumbnail' ) and has_post_thumbnail() ) {
        $featured_image = wp_get_attachment_image_src( get_post_thumbnail_id(), 'post-thumbnail' );
    } elseif ( function_exists( 'get_post_thumbnail_src' ) ) {
        $featured_image = get_post_thumbnail_src();
        if ( preg_match( '|^<img src="([^"]+)"|', $featured_image[0], $m ) )
            $featured_image[0] = $m[1];
    } else {
        $featured_image = false;
    }

    if ( ! empty( $featured_image ) ) {
        echo "\t" . '<enclosure url="' . $featured_image[0] . '" />' . "\n";
    }
    
}

add_action( 'rss2_item', 'add_featured_image_in_rss' );

Hello,
I use the add_featured_image_in_rss function ...
When I use it in the function-file of my theme it works correct, but when I use it as a plugin through the mu-plugins-folder, it adds a blank line in front of the xml-declaration …
Any idea how to solve that?

Greetings!