WordPress plugin used to allow attachment download with one click.

attachment.php

<?php
/**
 * Force attachments download :
 *
 * In order for this to work, when you insert the file into your post through the media uploader,
 * you have to select Post URL rather than File URL in the Link URL field before inserting into your post.
 * A link to a filename will follow the browser's preferences, but by linking to the WP post link,
 * you can control its behaviour.)
 *
 * @link       https://gist.github.com/smeric/cb4520e848aaa0be703c
 * @version    1.0.0
 * @package    downloadable-attachments
 * @author     Sébastien Méric <sebastien.meric@gmail.com>
 */

if ( have_posts() ) :
	while ( have_posts() ) :
		the_post();

		// Get attachment informations
		$uploads_dir = wp_upload_dir();
		$attachment_src = get_post_meta( $post->ID, '_wp_attached_file', true );
		$file = path_join( $uploads_dir['basedir'], $attachment_src );
		$filename = basename( get_attached_file( $post->ID ) );

		$filetype_and_ext = wp_check_filetype_and_ext( $file, $filename );

		$filename = $post->post_title
			? sanitize_title( $post->post_title ) . '.' . $filetype_and_ext['ext']
			: $filetype_and_ext['proper_filename']
				? $filetype_and_ext['proper_filename']
				: $filename;
		//$mime_type = get_post_mime_type( $post->ID );
		$mime_type = $filetype_and_ext['type']
			? $filetype_and_ext['type']
			: 'application/octet-stream';

		// Return file to download
		error_reporting( 0 ); // Errors may corrupt download
		ob_start();

		header( "Content-Description: File Download" );
		header( "Pragma: public" ); // Required
		header( "Expires: 0" );
		header( "Cache-Control: must-revalidate, post-check=0, pre-check=0" );
		header( "Cache-Control: private", false ); // Required for certain browsers 
		//header( "Content-Type: application/octet-stream" );
		header( "Content-Type: $mime_type" );
		header( "Content-Disposition: attachment; filename=\"$filename\";" );
		header( "Content-Transfer-Encoding: binary" );
		//header( "Content-Length: " . filesize( $file ) );

		ob_clean();
		ob_end_flush();
		readfile( $file );

		exit;
	endwhile;
endif;
?>

downloadable-attachments.php

<?php
/**
 * Downloadable attachments
 *
 * Use this plugin if you want that the attachment page link click triggers media download.
 *
 * @link       https://gist.github.com/smeric/cb4520e848aaa0be703c
 * @version    1.0.0
 * @package    downloadable-attachments
 * @author     Sébastien Méric <sebastien.meric@gmail.com>
 *
 * @wordpress-plugin
 * Plugin Name:       Downloadable attachments
 * Plugin URI:        https://gist.github.com/smeric/cb4520e848aaa0be703c
 * Description:       Use this plugin if you want that the attachment page link click triggers media download.
 * Version:           1.0.0
 * Author:            Sébastien Méric
 * Author URI:        http://www.sebastien-meric.com/
 * License:           GPL-2.0+
 * License URI:       http://www.gnu.org/licenses/gpl-2.0.txt
 * Text Domain:       downloadable_attachments
 * Domain Path:       /languages
 * 
 * This program is free software; you can redistribute it and/or modify it under the terms of the GNU
 * General Public License as published by the Free Software Foundation; either version 2 of the License,
 * or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without
 * even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 *
 * You should have received a copy of the GNU General Public License along with this program; if not, write
 * to the Free Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
 */

if ( ! defined( 'ABSPATH' ) ) exit; // Exit if accessed directly

/**
 * downloadable_attachments_template_include
 *
 * Inckude custom attachment template instead of theme one.
 * 
 * @since      1.0.0
 * @package    downloadable-attachments
 * @author     Sébastien Méric <sebastien.meric@gmail.com>
 **/
add_filter( 'template_include', 'downloadable_attachments_template_include' );
function downloadable_attachments_template_include( $original_template ) {
	$new_template = $original_template;

	if ( is_attachment() ) {
		$new_template = plugin_dir_path( __FILE__ ) . 'attachment.php';
	}

	return $new_template;
}