How to Include Custom Post Content From Plugin

Template filters have to return the name of a template file, not content. Their purpose is to override template selection with a different template.

If you want to modify the content output within the content of a template, then you need to add a filter to the_content.

function my_the_content_filter( $content ) {
    global $post;
    if ($post->post_type == "searchable-media"){
        // your code to append your meta fields to $content
    }
    return $content;
}

add_filter( 'the_content', 'my_the_content_filter' );