Add and save media attachment custom fields in WordPress

gistfile1.txt

/*
 * @param array $form_fields
 * @param object $post
 *
 * @return array
 */
function tenup_attachment_fields_to_edit ( $form_fields, $post )
{

    $form_fields[ "use_as_header_img" ] = array(
        'label' => __ ( "Use as header", '10up' ),
        'input' => 'text',
        'value' => get_post_meta ( $post->ID, 'use_as_header_img', TRUE ),
        'helps' => 'Enter "left" for the left header image, "right" for a right one,
        or leave blank to not use this image in the header.'
    );

    return $form_fields;
}

add_filter ( "attachment_fields_to_edit", "tenup_attachment_fields_to_edit", NULL, 2 );

/*
 * @param array $post
 * @param array $attachment
 *
 * @return array
 */
function tenup_attachment_fields_to_save ( $post, $attachment )
{

    if ( isset( $attachment[ 'use_as_header_img' ] ) )
        update_post_meta ( $post[ 'ID' ], 'use_as_header_img', $attachment[ 'use_as_header_img' ] );

    return $post;
}

add_filter ( 'attachment_fields_to_save', 'tenup_attachment_fields_to_save', NULL, 2 );