How to set boundary while using XmlHttpRequest and FormData
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I am trying to set the boundary correctly in the header while using FormData to post the XmlHttpRequest:

    xhr.open("POST",url);
    xhr.setRequestHeader("Content-type","multipart/form-data; boundary=...");
    
    var formData = new FormData();
    formData.append("filename", inputId.files[0]);
    formData.append(...);
    
    xhr.send(formData);

How do I get the boundary to be set in the request header here. I saw the request being set, the boundary is somehow created in the request. But the server has no idea on how to interpret it.