How do I POST attachments to Insight objects with JavaScript?

[Mathis Hellensberg]

I am trying to add a file to an Insight object with JavaScript. The POST request is working perfectly fine in Postman but my function returns a HTTP 500 Error :(

function post() { 
    var data = new FormData(); 
    data.append("file", "C:\\Users\\mhellensb\\Downloads\\image.png"); 
    data.append("encodedComment", "test");

    var xhr = new XMLHttpRequest(); 
    xhr.withCredentials = false;

    xhr.addEventListener("readystatechange", function () { 
        if (this.readyState === 4) { 
           console.log(this.responseText); 
        } 
    });

    xhr.open("POST", "http://localhost:8081/rest/insight/latest/attachments/object/7"); 
    xhr.setRequestHeader("X-Atlassian-Token", "no-check"); 
    xhr.setRequestHeader("Access-Control-Allow-Origin", "*"); 
    xhr.setRequestHeader("Authorization", "Basic eW91Omxvb2tMaWtlQW5JZGlvdA=="); 
    xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded"); 
    xhr.setRequestHeader("cache-control", "no-cache"); 
    xhr.send(data);
}