Doesnt save custom field selectbox data in media-frame

As I descriped earlier here:
http://wordpress.org/support/topic/doesnt-save-custom-field-data-in-media-frame

The issue was fixed but a new problem appeared:

If I do not touch the values in my custom fields it does not save the value of the first option of the selectbox.

The code for my selectbox is here:

/**
 * Form fields
 */
function custom_attachment_fields_to_edit( $form_fields, $post ) {
	$form_fields['size'] = custom_attachment_fields_to_edit_cb( $post );

	return $form_fields;
}
add_action( 'attachment_fields_to_edit', 'custom_attachment_fields_to_edit', 10, 2 );

/**
 * Form field callback
 */
function custom_attachment_fields_to_edit_cb( $post ) {
	$out_array = array(
		'one-col' => __( 'One column' ),
		'two-cols' => __( 'Two columns' ),
		'full-row' => __( 'Full row')
	);

	$out = '<select id="attachments-' . $post->ID . '-size" name="attachments[' . $post->ID . '][size]">';

	foreach ( $out_array as $k => $v ) {
		$selected = selected( $k, get_post_meta( $post->ID, 'attachment_size', true ), false );
		$out .= '<option value="' . $k . '"' . $selected .'>' . $v . '</option>';
	}

	$out .= '</select>';

	return array(
		'label' => __( 'Size' ),
		'input' => 'html',
		'html'  => $out,
	);
}

/**
 * Save our gallery meta
 */
function custom_attachment_fields_to_save( $post, $attachment ) {
	if( isset( $attachment['size'] ) )
		update_post_meta( $post['ID'], 'attachment_size', $attachment['size'] );

	return $post;
}
add_filter( 'attachment_fields_to_save', 'custom_attachment_fields_to_save', 10, 2 );