passes post_type as an array to the query string

cs_searchform.php

<?php
?>
<form action="<?php echo home_url(); ?>" id="topsearchform" method="get">
     <fieldset class="topsearchform">
         <label for="s" class="screen-reader-text"><?php _e('Search for:', 'curated'); ?></label>
         <input type="search" id="s" name="s" placeholder="" required />
         <input type="hidden" name="post_type[]" value="post" />
         <input type="hidden" name="post_type[]" value="song" />
         <input type="hidden" name="post_type[]" value="artist" />
         <input type="image" id="searchsubmit" alt="<?php _e('Search', 'curated'); ?>" src="<?php echo get_template_directory_uri(); ?>/images/searchicon.png" class="searchicon" />
     </fieldset>
</form>

this is the search page template that shows these search results https://gist.github.com/jazzsequence/5a2e82f9db8b29366ba3