JSFiddle - React, Tailwind, and code Playground

HTML

<input type="file" name="afile" id="afile" />

<a href="#" id="trigger">Click to fire off a POST XHR that absorbs its data via a populated FormData object</a>

CSS

a { display:block; margin: 20px 0; }

JavaScript

function fireOffFormData(e){ 

    var formData = new FormData();
    formData.append("username", "Groucho");
    formData.append("accountnum", 123456);
    formData.append("afile", document.querySelector('#afile').files[0] );

    var xhr = new XMLHttpRequest();
    xhr.open('POST', '/_save/', true); 
    xhr.send(formData);
    
}


document.querySelector('#trigger').addEventListener('click', fireOffFormData, false);