trouble with file_mime_type() in upload.php

    I've uploaded an xls file and I seem to be getting the wrong mime type for it. 

This is the line of trouble:

$mime = @finfo_file($finfo, $file['tmp_name']);

The $mime says that it is a "text/plain;charset=utf-16le" but the screenshot attached shows that it is actually a "vnd.ms-excel". After this it is all downhill. A few lines down (line 1241 of upload.php)

if (is_string($mime) && preg_match($regexp, $mime, $matches))
{
  $this->file_type = $matches[1];
  return;
}
This causes the file_type to be text when I am only allowing XLS | XLSX. 

Any ideas on what to do? 

proof that an old dog can learn new tricks