Excel (.xls) files detected as application/x-msi

[iffa]

Related to #162, #225

.xls files seem to be detected as msi files, even after msi false-positive fixes. Confirmed on files from Office 2016+, Mac & Windows.

See files: https://drive.google.com/open?id=1OqynCO-kHMolp4QFkiS3dAiBfJ58DYXi

[sindresorhus]

From the readme:

The following file types will not be accepted:

• .xls - Too old and difficult to parse.

[iffa]

Totally missed that note, my bad.

[msageryd]

I'm migrating to this library from my old home cooked file checking lib. I had a check for old xls files. Was it flawed? Otherwise, please integrate this test into your lib.

//Old XLS files
function isExcel(buf) {
  if (!buf || buf.length < 520) {
    return false;
  }

  return (
    buf[512] === 0x09 &&
    buf[513] === 0x08 &&
    buf[514] === 0x10 &&
    buf[515] === 0x00 &&
    buf[516] === 0x00 &&
    buf[517] === 0x06 &&
    buf[518] === 0x05 &&
    buf[519] === 0x00
  );
}