Why do xls documents have such a mimetype?

Question

Node.js

Allowed types

let allowedMimetypes = [
    'image/jpeg',
    'image/jpg',
    'image/png',
    'image/gif',
    'application/pdf',
    'application/zip',
    'application/x-rar-compressed',
    'application/vnd.ms-excel',
    'application/vnd.ms-office',
    'application/vnd-xls',
    'application/msword',
    'application/doc',
    'application/vnd.msword',
    'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet',
    'application/xls',
    'application/x-xls',
    'text/plain',
    'application/vnd.ms-excel',
    'application/msexcel',
    'application/x-msexcel',
    'application/x-ms-excel',
    'application/x-excel',
    'application/x-dos_ms_excel',
    'application/excel'
];

xlsx passes fine. But xls does not want to. When checking the mimetype, it gives "application/octet-stream", which is just a binary file. What could it be?

Answer 1

xlsx is a compressed xml whose dtd can be specifically identified
and xls has no markup structure