<input type="file" multiple="multiple" accept=".txt,image/*">
<a href="#" class="upload_files button">Загрузить файлы</a>

 var files;
document.querySelector('input[type=file]').addEventListener('change', e => files = (e.target.files));
document.querySelector('.upload_files').addEventListener('click', e => {
  if (typeof files == 'undefined') return;
  var data = new FormData();
  Array.from(files).forEach((file, index) => {
      data.append(index, file)
  });
data.append('my_file_upload', 1);
fetch('./submit.php', {
    method: 'POST',
      body: data,
 })
........
if (isset($_POST['my_file_upload'])) {

    $uploaddir = './uploads';

    if (!is_dir($uploaddir)) mkdir($uploaddir, 0777);

    $files = $_FILES;
    $done_files = array();


    foreach ($files as $file) {
        $file_name = $file['name'];

        if (move_uploaded_file($file['tmp_name'], "$uploaddir/$file_name")) {
            $done_files[] = realpath("$uploaddir/$file_name");
        }
    }

    $data = $done_files ? array('files' => $done_files) : array('error' => 'Ошибка загрузки файлов.');

    echo(json_encode($data));
}