Send a blob (which may be a file) from client app

send-blob.js

// Blob may contain something else, check https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/send#Syntax
// Keep in mind this requires preflight
const blob = new Blob(['this is a test'], { type: 'application/x-www-form-urlencoded'})
const xhr = new XMLHttpRequest();

xhr.open("POST", 'http://localhost:8080/files', true);

xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");

xhr.onreadystatechange = function() {
  if (this.readyState === XMLHttpRequest.DONE && (this.status === 200 || this.status === 201)) {
    console.log('Blob uploaded')
  }
}
xhr.setRequestHeader('header-name', 'header-value');
xhr.send(blob);