PHP打开.msg文件

Some users are uploading .msg files to a folder. I need to be able to open it up in a separate window or just open the original file.

I can open everything that I allow, just not .msg files.

Here is the code I'm using to display and download:

<?php
  function listFolderFiles($dir){
    $ffs = scandir($dir);

    foreach($ffs as $ff){
      if($ff != '.' && $ff != '..'){
        if(is_dir($dir.'/'.$ff)){
          // just displaying headers here
        }
        else{
          $filesize = filesize($dir . '/' . $ff);
          echo "<a download href='$dir/$ff'>$ff</a>";
        }
      }
    }
    echo "<br />";
 }
 listFolderFiles('files');
 ?>

Not sure if I should post the code I use to upload, but I did include the .msg extension, and the user is indeed able to upload .msg files.

I found this post: display .msg file in browser using php

And the answer there advised to use various HEADERS. But I'm not sure why I should place the code for the headers.