function add_image_attachment_fields_to_edit($form_fields, $post) {
  // $form_fields is a an array of fields to include in the attachment form
  // $post is nothing but attachment record in the database
  //     $post->post_type == 'attachment'
  // attachments are considered as posts in WordPress. So value of post_type in wp_posts table will be attachment
  // now add our custom field to the $form_fields array
  // input type="text" name/id="attachments[$attachment->ID][custom1]"
$categories = get_categories();
//var_dump($categories);
$form_fields["custom4"]["label"] = __("Custom Select");
$form_fields["custom4"]["input"] = "html";
$form_fields["custom4"]["html"] = "<select name='attachments[{$post->ID}][custom4]' id='attachments[{$post->ID}][custom4]'>";

foreach ($categories as $category) {
     $form_fields["custom4"]["html"] .= '<option value="$category">' . $category . '</option>';
}

$form_fields["custom4"]["html"] .= "</select>";
   return $form_fields;
}
// now attach our function to the hook
add_filter("attachment_fields_to_edit", "add_image_attachment_fields_to_edit", null, 2);

function add_image_attachment_fields_to_save($post, $attachment) {
  // $attachment part of the form $_POST ($_POST[attachments][postID])
        // $post['post_type'] == 'attachment'
  if( isset($attachment['restaurant']) ){
    // update_post_meta(postID, meta_key, meta_value);
    update_post_meta($post['ID'], '_restauarant', $attachment['restaurant']);
  }
  return $post;
}