title: C# - convert xls file to xlsx without office components
body: <p>I have a server that can't have any office installed on it, so I'm using ClosedXML to do some manipulations on excel files.
  But ClosedXML only works on .xlsx file and I also have xls file to handle.
  I spend almost a full day searching for a way to convert .xls files to .xlsx files without using any office libraries (since there is no office installed on my designated server...)</p>
  
  <p>Can someone PLEASE tell me how can I convert these .xls files to .xlsx files ?
  I can't use Microsoft.Office.Interop because it requires having office installed on the server.</p>