Working code:

// Add custom column with file size info to attachment page
add_filter( \'manage_media_columns\', \'bb2_manage_media_columns\', 10, 2 );
function bb2_manage_media_columns( $columns )
{
    $columns[\'filesize\'] = __( \'Storlek\', \'bb2\' );
    return $columns;
}

add_action( \'manage_media_custom_column\', \'bb2_manage_media_custom_column\', 10, 2 );
function bb2_manage_media_custom_column( $column_name, $id )
{
    switch ( $column_name )
    {
        case \'filesize\' :
            $bytes = filesize(get_attached_file($id));
            echo size_format($bytes);
        break;

        default :
        break;
    }
}