JSFiddle - React, Tailwind, and code Playground

JavaScript

form.onsubmit = function(event) {
  event.preventDefault();

  // Update button text.
  uploadButton.innerHTML = 'Uploading...';
  var files = fileSelect.files;
  var formData = new FormData();
  for (var i = 0; i < files.length; i++) {
    var file = files[i];

    // Check the file type.
    if (!file.type.match('image.*')) {
      continue;
    }

    // Add the file to the request.
    formData.append('file[]', finalFiles, file.name);
  }
  // Files
  formData.append(name, finalFiles, filename);
  // Set up the request.
  console.log(file);
  var xhr = new XMLHttpRequest();
  xhr.open('POST', 'http://zayavlenie.com.ua/mail-dev.php', true);
  // The rest of the code will go here...
  // Set up a handler for when the request finishes.
  xhr.onload = function() {
    if (xhr.status === 200) {
      // File(s) uploaded.
      uploadButton.innerHTML = 'Отправка';
    } else {
      alert('An error occurred!');
    }
  };
  xhr.send(formData);
}