First Image in Post as Thumbnail (easy-add-thumbnail)

<?php
/*
* Plugin Name: Easy Add Thumbnail
* Plugin URI: http://wordpress.org/extend/plugins/easy-add-thumbnail/
* Description: Checks if you defined the featured image, and if not it sets the featured image to the first uploaded image into that post. So easy like that...
* Author: Samuel Aguilera
* Version: 1.1.1
* Author URI: http://www.samuelaguilera.com
* Requires at least: 3.7
*/

/*
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License version 3 as published by
the Free Software Foundation.

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.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

if ( function_exists( 'add_theme_support' ) ) {

add_theme_support( 'post-thumbnails' ); // This should be in your theme. But we add this here because this way we can have featured images before swicth to a theme that supports them.

function easy_add_thumbnail($post) {

    $already_has_thumb = has_post_thumbnail();
    $post_type = get_post_type( $post->ID );
    $exclude_types = array('');
    $exclude_types = apply_filters( 'eat_exclude_types', $exclude_types );

    // do nothing if the post has already a featured image set
    if ( $already_has_thumb ) {
        return;
    }

    // do the job if the post is not from an excluded type
    if ( ! in_array( $post_type, $exclude_types ) )  {

        // get first attached image
        $attached_image = get_children( "order=ASC&post_parent=$post->ID&post_type=attachment&post_mime_type=image&numberposts=1" );

        if ( $attached_image ) {

            $attachment_values = array_values( $attached_image );
            // add attachment ID
            add_post_meta( $post->ID, '_thumbnail_id', $attachment_values[0]->ID, true );

        }


    }

}

// set featured image before post is displayed (for old posts)
add_action('the_post', 'easy_add_thumbnail');

// hooks added to set the thumbnail when publishing too
add_action('new_to_publish', 'easy_add_thumbnail');
add_action('draft_to_publish', 'easy_add_thumbnail');
add_action('pending_to_publish', 'easy_add_thumbnail');
add_action('future_to_publish', 'easy_add_thumbnail');

}

?>