title: Apache poi creates Excel file with error
body: <p>I am making an Excel file Using Apache_poi. This file will be created on the action event of a button in my Swing application. The Excel sheet gets created just fine but, when I open it, I get this error:</p>
  
  <blockquote>
    <p>The file you are trying to open is in different format than specified by the file extension</p>
  </blockquote>
  
  <p>and when I open the file it is completely empty.</p>
  
  <pre><code>private void jButton3ActionPerformed(java.awt.event.ActionEvent evt) {                                         
      try {
          // String personalclass=jComboBox1.getSelectedItem().toString();
          // String ac="academic";
          // FileOutputStream output=new FileOutputStream("D:\\"+personalclass+ac+".xls");
          FileOutputStream output=new FileOutputStream("D:\\workbook.xls");
          Workbook wb=new HSSFWorkbook();
          Sheet sheet =wb.createSheet("Class 1"); 
          Cell cell=sheet.createRow(0).createCell(3);
          cell.setCellValue("Thisisatestofmerging");
          //sheet.addMergedRegion(new CellRangeAddress(1, 1, 1, 2));
          output.close();
      }
      catch(Exception e)
      {
      }
  }  
  </code></pre>