title: Excel "found unreadable content" error opening a xlsx created from C#
body: <p>I'm creating an Excel file (xlsx) from C# using OpenXML.</p>
  
  <p>When I create a xlsx file and I open, it has no error.</p>
  
  <p>When I try to attach that file on an e-mail and send it, Excel returns the message "found unreadable content" opening the attached file, with Yes/No button to continue.</p>
  
  <p>Clicking on Yes the file is opened correctly.</p>
  
  <p>This is my code to attach an xlsx file:</p>
  
  <pre><code>byte[] bt = functionToCreateXlsxfile();
  
  MailMessage message = new MailMessage();
  
  System.Net.Mail.Attachment attach = new System.Net.Mail.Attachment(
          new MemoryStream(bt),
          "myfile.xlsx", 
          "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet");
  
  attach.ContentDisposition.CreationDate = DateTime.Now;
  attach.ContentDisposition.ModificationDate = DateTime.Now;
  attach.ContentDisposition.Inline = false;
  attach.ContentDisposition.Size = bt.Length;
  
  message.Attachments.Add(attach);
  </code></pre>