Saving attachment custom fields

Got it:

add_filter("attachment_fields_to_edit", "my_image_attachment_fields_to_edit", null, 2);
function my_image_attachment_fields_to_edit($form_fields, $post) {

    $form_fields["OwnersHeading"]["tr"] = "
    <tr>
        <td colspan='2' style="font-size:16px;padding-left:15px">Tag Users:</td>
    </tr>";

    // get list of users
    $wp_user_query = new WP_User_Query( array( 'orderby' => 'display_name' ) );
    $authors = $wp_user_query->get_results();

    if (!empty($authors)) { 
        foreach ($authors as $author) {

            $belongs_to_value = (bool)get_post_meta( $post->ID, "_owner_$author->ID", true );

            // create the field
            $form_fields["owner_$author->ID"] = array(
                'input' => 'html',
                'html'  => '
                    <input type="checkbox" id="attachments-' . $post->ID . '-owner'.$author->ID.'" name="attachments[' . $post->ID . '][owner_'.$author->ID.']" value="1"' . ( $belongs_to_value ? ' checked="checked"' : '' ) . ' />
                    <label for="attachments-' . $post->ID . '-owner'.$author->ID.'">'.$author_info->first_name.' '.$author_info->last_name.'</label>',
            );

        }
    }

    return $form_fields;
}


add_filter("attachment_fields_to_save", "my_image_attachment_fields_to_save", null, 2);
function my_image_attachment_fields_to_save($post, $attachment) {

    // get list of users
    $wp_user_query = new WP_User_Query( array( 'orderby' => 'display_name' ) );
    $authors = $wp_user_query->get_results();

    if (!empty($authors)) {
        foreach ($authors as $author) {
            update_post_meta($post['ID'], '_owner_'.$author->ID.'', $attachment['owner_'.$author->ID.'']);      
        }
    }

    return $post;
}