this points to null in a callback in Javascript

As per Nested functions & the "this" keyword in JavaScript & https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this

I understood that when a function is called directly(i.e.not with an object) , this should point to the global object.

In the below Node code , after the comment "//why is this null??" i thought "this" should be pointing to the global object. But it instead is "null"...why?

'use strict';

var fs = require('fs');

function FileThing() {
  this.name = '';

  this.exists = function exists(cb) {
    console.log('Opening: ' + this.name);
    fs.open(this.name, 'r', function onOpened(err, fd) {
      if (err) {
        if (err.code === 'ENOENT') { // then file didn't exist
          //why is this null??
          console.log(this);
          console.log('Failed opening file: ' + this.name);
          return cb(null, false);
        }
        // then there was some other error
        return cb(err);
      }
      fs.close(fd);
      return cb(null, true);
    });
  };
}

var f = new FileThing();
f.name = 'thisFileDoesNotExist';
f.exists(function onExists(err, exists) {
  if (err) {
    return console.log('ERROR! ' + err);
  }
  console.log('file ' + (exists ? 'DOES' : 'does NOT') + ' exist');
});