title: Is it possible to add or modify multipart/form-data headers to emulate a <input type="file"> element?
body: <p>Specifically I'd like to be able to set the <code>Content-Disposition</code> and <code>Content-Type</code> headers for a specific input field to look like a type="file" input field, e.x.</p>
  
  <p><code>Content-Disposition: form-data; name="myformfile"; filename="myfile.jpeg"
  Content-Type: image/jpeg</code></p>
  
  <p>I want to do this on an existing HTML form, not by manually formatting an XMLHttpRequest body.</p>
  
  <p>(The use case is in a browser extension / userscript, to replace a file upload button with one the extension can programmatically set)</p>