ajax does not work for this becuase there is little you can do with the response data (downloaded file).

   success: function(data) { ...

not much you can do with data. a better approach is a form post to a tarhget window:

   <form action="@Url.Content("~/ExcelReport")" method="post" target="_blank" id="report">
      <input type="hidden" name="__RequestVerificationToken" value="@token"/>
      <input type="hidden" name="content" />
      <input type="hidden" name="fileName" value="export.xls"/>
   </form>

   <script>
        var frm = $('#report'):
        frm.find('name=[content]').val($('#grid-fixo').html());
        frm.submit();