functions.php

<?php

/**
 * Hidden fields on media uploader
 *
 * @param $post array, the post data for database
 * @param $attachment array, attachment fields from $_POST form
 * @return $post array, modified post data
 */
 
function be_attachment_hidden_field_save( $post, $attachment ) {
	update_post_meta( $post['ID'], 'be_show_this', 'true' );
	return $post;
}

add_filter( 'attachment_fields_to_save', 'be_attachment_hidden_field_save', 10, 2 );