title: "Excel found unreadable content" warning when opening Excel files made with PHPExcel
body: <p>I'm trying to download Excel files (xlsx) using <a href="http://download.codeplex.com/Project/Download/FileDownload.aspx?ProjectName=phpexcel&amp;DownloadId=212184&amp;FileTime=129433274033800000&amp;Build=17831" rel="noreferrer"><code>PHPExcel</code></a> as follows.</p>
  
  <pre><code>require_once("../../phpExcel/Classes/PHPExcel.php");
  require_once("../../phpExcel/Classes/PHPExcel/IOFactory.php");
  
  $objPHPExcel = new PHPExcel();
  
  $objPHPExcel-&gt;getProperties()-&gt;setCreator("Tiny")
  -&gt;setLastModifiedBy("Tiny")
  -&gt;setTitle("Office 2007 XLSX Test Document")
  -&gt;setSubject("Office 2007 XLSX Test Document")
  -&gt;setDescription("Test document for Office 2007 XLSX, generated using PHP classes.");
  
  $objPHPExcel-&gt;setActiveSheetIndex(0);
  $sheet=$objPHPExcel-&gt;getActiveSheet();
  $sheet-&gt;setCellValue('A1', 'Hello');
  $sheet-&gt;setCellValue('B2', 'world!');
  $sheet-&gt;setCellValue('C1', 'Hello');
  $sheet-&gt;setCellValue('D2', 'world!');
  
  header('Content-Type: application/vnd.openxmlformats-officedocument.spreadsheetml.sheet');
  header('Content-Disposition: attachment;filename="test.xlsx"');
  header('Cache-Control: max-age=0');
  
  $objWriter = PHPExcel_IOFactory::createWriter($objPHPExcel, 'Excel2007');
  $objWriter-&gt;save('php://output');
  </code></pre>
  
  <p>It works but when I try to open the Excel file, it asks for a confirmation in a conform dialog.</p>
  
  <blockquote>
    <p>Excel found unreadable content in 'test.xlsx'. Do you want to recover
    the contents of this workbook? If you trust the source of this
    workbook, click Yes.</p>
  </blockquote>
  
  <p>I also tried to change the header as follows</p>
  
  <pre><code>header("Pragma: public");
  header("Expires: 0");
  header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
  header("Content-Type: application/force-download");
  header("Content-Type: application/octet-stream");
  header("Content-Type: application/download");;
  header("Content-Disposition: attachment;filename=test.xlsx");
  header("Content-Transfer-Encoding: binary");
  </code></pre>
  
  <p>It still asks for the same confirmation. What am I doing wrong here, please?</p>