Difference between AJAX request and form HTTP request

Question

I am follwing the below link to download a file in chrome browser

Streaming large files in a java servlet

But the problem is that the file is getting downloaded when submit button in form is clicked, but if I send ajax request the file is not getting downloaded. I am not able to understand the problem. This is how my ajax request looks:

$.ajax({
    type : "POST",
    url : url,
    dataType : "json",

});

Any help will be appreciated. Thanks!