filename1 => [field_name1, start_position, length ],
             [field_name2, start_position, length ],
                  ..             ..          ..
             [field_namen, start_position, length ]
filename2 => [field_name1, start_position, length ],
             [field_name2, start_position, length ],
                  ..             ..          ..
             [field_namen, start_position, length ]