Adds custom field to WordPress 3.5 Media Manager. The field's unique key is 'meta_link'.

media-gallery-custom.php

<?php
// In your functions.php

/**
 * Add custom field
*/
function mytheme_attachment_fields( $fields, $post ) {

    $meta = get_post_meta($post->ID, 'meta_link', true);
    $fields['meta_link'] = array(
        'label' => 'Video URL',
        'input' => 'text',
        'value' => $meta,
        'show_in_edit' => true,
    );
    return $fields;
}
add_filter( 'attachment_fields_to_edit', 'mytheme_attachment_fields', 10, 2 );

/**
 * Update custom field on save
*/
function mytheme_update_attachment_meta($attachment){
    global $post;
    update_post_meta($post->ID, 'meta_link', $attachment['attachments'][$post->ID]['meta_link']);
    return $attachment;
}
add_filter( 'attachment_fields_to_save', 'mytheme_update_attachment_meta', 4);

/**
 * Update custom field via ajax
*/
function mytheme_media_xtra_fields() {
    $post_id = $_POST['id'];
    $meta = $_POST['attachments'][$post_id ]['meta_link'];
    update_post_meta($post_id , 'meta_link', $meta);
    clean_post_cache($post_id);
}
add_action('wp_ajax_save-attachment-compat', 'mytheme_media_xtra_fields', 0, 1);