Attaching a file already in place

gistfile1.php

<?php

$url = home_url().'/wp-content/uploads/yourfilehere.jpg';
$type = 'image/jpeg';
$file = ABSPATH.'/wp-content/uploads/yourfilehere.jpg';
$title = preg_replace('/\.[^.]+$/', '', basename($file));
$content = '';
$parent_id = 0;

// use image exif/iptc data for title and caption defaults if possible
if ( $image_meta = @wp_read_image_metadata($file) ) {
	if ( trim( $image_meta['title'] ) && ! is_numeric( sanitize_title( $image_meta['title'] ) ) )
		$title = $image_meta['title'];
	if ( trim( $image_meta['caption'] ) )
		$content = $image_meta['caption'];
}

if ( isset( $desc ) )
	$title = $desc;

// Construct the attachment array
$attachment = array(
	'post_mime_type'	=>	$type,
	'guid'			=>	$url,
	'post_parent'		=>	$parent_id,
	'post_title'		=>	$title,
	'post_content'		=>	$content
);

// Save the attachment metadata
$id = wp_insert_attachment($attachment, $file, $parent_id);
if ( !is_wp_error($id) )
	wp_update_attachment_metadata( $id, wp_generate_attachment_metadata( $id, $file ) );