Index.cshtml

<form class="form-horizontal" method="post" asp-action="Upload" asp-controller="Upload" enctype="multipart/form-data">
  <label class="col-md-4 control-label" for="arquivo">Arquivo</label>
  <div class="col-md-4">
    <input type="file" id="arquivo" name="arquivo"/>
  </div>
  <button class="btn btn-primary"> Upload </button>
</form>