FormData插入额外的TExt

Question

I'm using form data in Javascript to send my inputs to the server but I noticed something funky. In Firefox and Chrome it sends back extra text along with the data and adds it to the original value. Here is an example of what chrome outputs

------WebKitFormBoundaryq5GElQTvd3mbU5qE--

Here is my code. I am doing a PUT request.

var formData = new FormData();
                formData.append('user','tester');
                var req = new XMLHttpRequest();

                req.onload=function(e){
                    console.log(this)
                }

                req.open('PUT','http://localhost/site_template/create_file.php',true);
                req.send(formData);