¿Cómo puedo agregar un campo de URL a la ventana de archivos adjuntos?

Uso un plugin muy aproximado para agregar información sobre el artista y una URL a los archivos multimedia. Necesita algunos ajustes (y yo necesito tiempo), pero funciona y puede demostrar cómo agregar los campos adicionales y cómo usarlos en su tema:

<?php
/*
Plugin Name: Toscho’s Media Artist Field
Description: Adds two field to attachments – Artist and Artist URL – and adds this information to captions.
Version:     0.1
Author:      Thomas Scholz
Author URI:  http://toscho.de
Created:     19.09.2010
*/
$Toscho_Media_Artist = new Toscho_Media_Artist(
    array (
        'artist_name' => array (
            'public' => 'toscho_artist_name'
        ,   'hidden' => '_toscho_artist_name'
        ,   'label'  => 'Fotograf (Name)'
        )
    ,   'artist_url' => array (
            'public' => 'toscho_artist_url'
        ,   'hidden' => '_toscho_artist_url'
        ,   'label'  => 'Fotograf (URL)'
        )
    )
,   'Foto: '
);
/**
 * Adds two fields for credits to any media file: name and URL.
 *
 * Based on the clear tutorial by Andy Blackwell:
 * @link http://net.tutsplus.com/?p=13076
 *
 * @author "Thomas Scholz"
 *
 */
class Toscho_Media_Artist
{
    public
        $fields = array (
            'artist_name' => array (
                'public' => 'toscho_artist_name'
            ,   'hidden' => '_toscho_artist_name'
            ,   'label'  => 'Artist Name'
            )
        ,   'artist_url' => array (
                'public' => 'toscho_artist_url'
            ,   'hidden' => '_toscho_artist_url'
            ,   'label'  => 'Artist URL'
            )
        )
        // Maybe its own field?
    ,   $caption_prefix
    ,   $br_before = TRUE;

    public function __construct(
        $fields         = array()
    ,   $caption_prefix = 'Source: '
    ,   $br_before      = TRUE
    )
    {
        $this->fields         = array_merge($this->fields, $fields);
        $this->caption_prefix = $caption_prefix;
        $this->br_before      = (bool) $br_before;

        $this->set_filter();
    }

    public function set_filter()
    {
        add_filter(
            'attachment_fields_to_edit'
        ,   array ( $this, 'add_fields' )
        ,   15
        ,   2
        );
        add_filter(
            'attachment_fields_to_save'
        ,   array ( $this, 'save_fields' )
        ,   10
        ,   2
        );
        add_filter(
            'img_caption_shortcode'
        ,   array ( $this, 'caption_filter' )
        ,   1
        ,   3
        );
    }

    public function add_fields($form_fields, $post)
    {
        foreach ( $this->fields as $field)
        {
            $form_fields[ $field['public'] ]['label'] = $field['label'];
            $form_fields[ $field['public'] ]['input'] = 'text';
            $form_fields[ $field['public'] ]['value'] = get_post_meta(
                $post->ID
            ,   $field['hidden']
            ,   TRUE
            );
        }
        return $form_fields;
    }

    public function save_fields($post, $attachment)
    {
        foreach ( $this->fields as $field)
        {
            if ( isset ( $attachment[ $field['public'] ]) )
            {
                update_post_meta(
                    $post['ID']
                ,   $field['hidden']
                ,   $attachment[ $field['public'] ]
                );
            }
        }

        return $post;
    }

    public function caption_filter($empty, $attr, $content = '')
    {
        /* Typical input:
         * [caption id="attachment_525" align="aligncenter"
         * width="300" caption="The caption."]
         * <a href="http://example.com/2008/images-test/albeo-screengrab/"
         * rel="attachment wp-att-525"><img
         * src="http://example.com/uploads/2010/08/albeo-screengrab4.jpg?w=300"*alt="" title="albeo-screengrab" width="300" height="276"
         * class="size-medium wp-image-525" /></a>[/caption]
         */
        extract(
            shortcode_atts(
                array (
                    'id'        => ''
                ,   'align'     => 'alignnone'
                ,   'width'     => ''
                ,   'caption'   => ''
                ,   'nocredits' => '0'
                )
            ,   $attr
            )
        );

        // Let WP handle these cases.
        if ( empty ($id ) or 1 == $nocredits )
        {
            return '';
        }

        if ( 1 > (int) $width || empty ( $caption ) )
        {
            return $content;
        }

        if ( ! empty ( $id ) )
        {
            // Example: attachment_525
            $html_id     = 'id="' . esc_attr($id) . '" ';
            $tmp         = explode('_', $id);
            $id          = end($tmp);

            $sub_caption = '';
            $artist_name = get_post_meta($id, $this->fields['artist_name']['hidden'], TRUE);
            $artist_url  = get_post_meta($id, $this->fields['artist_url']['hidden'], TRUE);

            // Okay, at least one value.
            if ( '' != $artist_name . $artist_url )
            {
                $sub_caption .= $this->br_before ? '<br />' : '';
                $sub_caption .= '<span class="media-artist">' . $this->caption_prefix;

                // No name given. We use the shortened URL.
                if ( '' == $artist_name )
                {
                    $sub_caption .= '<a rel="author" href="'
                        . $artist_url . '">'
                        . $this->short_url($artist_url)
                        . '</a>';
                } // We have just the name.
                elseif ( '' == $artist_url )
                {
                    $sub_caption .= $artist_name;
                } // We have both.
                else
                {
                    $sub_caption .= '<a rel="author" href="'
                        . $artist_url . '">'
                        . $artist_name
                        . '</a>';
                }

                $sub_caption .= '</span>';
            }

            $caption .= $sub_caption;
        }

        return '<div ' . $html_id . 'class="wp-caption ' . esc_attr($align)
        . '" style="width: ' . (10 + (int) $width) . 'px">'
        . do_shortcode( $content ) . '<p class="wp-caption-text">'
        . $caption . '</p></div>';
    }

    public function short_url($url, $max_length=20)
    {
        $real_length = mb_strlen($url, 'UTF-8');

        if ( $real_length <= $max_length )
        {
            return $url;
        }

        $keep = round( $max_length / 2 ) - 1;

        return mb_substr($url, 0, $keep, 'UTF-8') . '…'
            . mb_substr($url, -$keep, $real_length, 'UTF-8');
    }
    # @todo uninstall
}

Respondiendo a la pregunta de Drew en los comentarios, puedes personalizar el HTML para el campo configurando input en una nueva cadena, y luego agregando esa misma cadena como clave a la matriz $form_fields .

De forma predeterminada, WordPress solo aceptará text y textarea para el tipo input . Cualquier otra cosa deberá definirse de manera personalizada como se muestra a continuación. No he intentado en realidad la persistencia de los campos de formulario de esta manera, por lo que para hacer otro tipo de entrada, como un botón de opción, podría requerir un poco más de delicadeza.

add_filter( 'attachment_fields_to_edit', 'change_fields', 10, 2 );
function change_fields( $form_fields, $post ) {

    $form_fields["some_new_field"] = array(  
            "label" => __("Type something"),
            "input" => "arbitrary_value",
            "value" => get_post_meta($post->ID, "some_new_field", true),
            "arbitrary_value" => "hello world!"
        );
}