WordPress - Add PDF filter to media library

gistfile1.php

<?php

// Add PDF filter to media library
add_filter( 'post_mime_types', 'modify_post_mime_types' );
function modify_post_mime_types( $post_mime_types ) {
	// select the mime type, here: 'application/pdf'
	// then we define an array with the label values
	$post_mime_types['application/pdf'] = array( __( 'PDFs' ), __( 'Manage PDFs' ), _n_noop( 'PDF <span class="count">(%s)</span>', 'PDFs <span class="count">(%s)</span>' ) );
	// then we return the $post_mime_types variable
	return $post_mime_types;
}

?>