title: What is the default Content-Type in XMLHttpRequest?
body: <p>I have created a <a href="http://umbrellajs.com/" rel="nofollow">javascript library</a> and I am setting manually the <a href="https://github.com/umbrellajs/umbrella/blob/master/umbrella.js#L367" rel="nofollow">Content-Type</a> right now. I would love to know what is the default <em>Content-Type</em>. The <a href="https://xhr.spec.whatwg.org/" rel="nofollow">XHR documentation</a> for <a href="https://xhr.spec.whatwg.org/#the-send%28%29-method" rel="nofollow">the send() method</a> is confusing at best:</p>
  
  <blockquote>
    <p><strong>4.</strong> If body is null, go to the next step. Otherwise ...</p>
    
    <blockquote>
      <p>BodyInit</p>
      
      <p>If body is a string, set encoding to <code>UTF-8</code>.</p>
      
      <p>Set request body and Content-Type to the result of extracting body.</p>
    </blockquote>
  </blockquote>
  
  <p>Where, <a href="https://fetch.spec.whatwg.org/#concept-bodyinit-extract" rel="nofollow">the result of extracting body</a> for a text is:</p>
  
  <ul>
  <li>Blob</li>
  <li>BufferSource</li>
  <li>FormData</li>
  <li>URLSearchParams</li>
  <li>USVString</li>
  </ul>
  
  <p>So, which of these types would it be? The <code>body</code> passed to <code>send()</code> is in the default, escaped format of <code>a=b&amp;c=d</code>. Is it a <code>URLSearchParams</code>? or a <code>USVString</code>? or a <code>Blob</code>? There's a description on each one but it's too cryptic for me to understand (I only know that it's probably not a <a href="https://developer.mozilla.org/en/docs/Web/API/FormData" rel="nofollow">FormData</a>).</p>
  
  <p>Note: please provide some official documentation if possible</p>