formupload.js

let dropArea = document.getElementById("drop-area");

['dragenter', 'dragover', 'dragleave', 'drop'].forEach(eventName => {
  dropArea.addEventListener(eventName, preventDefaults, false)   
  document.body.addEventListener(eventName, preventDefaults, false)
});

['dragenter', 'dragover'].forEach(eventName => {
  dropArea.addEventListener(eventName, highlight, false)
});

['dragleave', 'drop'].forEach(eventName => {
  dropArea.addEventListener(eventName, unhighlight, false)
});

dropArea.addEventListener('drop', handleDrop, false)

function preventDefaults (e) {
  e.preventDefault()
  e.stopPropagation()
}

function highlight(e) {
  dropArea.classList.add('highlight')
}

function unhighlight(e) {
  dropArea.classList.remove('active')
}

function handleDrop(e) {
  var dt = e.dataTransfer
  var files = dt

  handleFiles(dt)
}

function handleFiles(input) {
  let file = input.files[0];

  if (file.name.slice(-5) == '.html') {
    let reader = new FileReader();

    reader.readAsText(file);

    reader.onload = function() {
      result = reader.result;
      alert(result);
    };

    reader.onerror = function() {
      er.innerHTML = reader.error;
    };
  } else {
    er.innerHTML = 'Неизвестный файл, поддерживаются только .html!';
  }
  //alert(`File name: ${file.name}`);
}