jquery-dropzone.html

<!-- HTML heavily inspired by http://blueimp.github.io/jQuery-File-Upload/ -->
<div class="table table-striped" class="files" id="previews">

  <div id="template" class="file-row">
    <!-- This is used as the file preview template -->
    <div>
        <span class="preview"><img data-dz-thumbnail /></span>
    </div>
    <div>
        <p class="name" data-dz-name></p>
        <strong class="error text-danger" data-dz-errormessage></strong>
    </div>
    <div>
        <p class="size" data-dz-size></p>
        <div class="progress progress-striped active" role="progressbar" aria-valuemin="0" aria-valuemax="100" aria-valuenow="0">
          <div class="progress-bar progress-bar-success" style="width:0%;" data-dz-uploadprogress></div>
        </div>
    </div>
    <div>
      <button class="btn btn-primary start">
          <i class="glyphicon glyphicon-upload"></i>
          <span>Start</span>
      </button>
      <button data-dz-remove class="btn btn-warning cancel">
          <i class="glyphicon glyphicon-ban-circle"></i>
          <span>Cancel</span>
      </button>
      <button data-dz-remove class="btn btn-danger delete">
        <i class="glyphicon glyphicon-trash"></i>
        <span>Delete</span>
      </button>
    </div>
  </div>

</div>

the first line has a duplicate class attribute. xoxo

see my Bootstrap 4 fork

You might also consider changing id="template" to a class.