Force Attachment Pages to Download File

<?php
if ( ! function_exists( 'scseed_force_attachment_download' ) ) {
    function scseed_force_attachment_download() {
        if ( ! is_attachment() ) {
            return;
        }
        $post      = get_post();
        $file_path = get_attached_file( $post->ID );
        if ( ! $file_path || ! file_exists( $file_path ) ) {
            return;
        }
        $mime_type = get_post_mime_type( $post->ID );
        $filename  = basename( $file_path );
        header( 'Content-Description: File Transfer' );
        header( 'Content-Type: ' . $mime_type );
        header( 'Content-Disposition: attachment; filename="' . $filename . '"' );
        header( 'Content-Length: ' . filesize( $file_path ) );
        header( 'Cache-Control: must-revalidate' );
        readfile( $file_path );
        exit;
    }
    add_action( 'template_redirect', 'scseed_force_attachment_download' );
}