title: OpenXml SpreadsheetDocument.Open(...) get exception - The main part is missing
body: <p>I have an xlsx doc with headers and it is located in my project path.
  I want to insert data using OpenXML but I get an exception while opening the file:</p>
  
  <pre><code>using (SpreadsheetDocument myDoc = SpreadsheetDocument.Open(MyxlsFileName, true))
  {
      // ...
  }
  </code></pre>
  
  <p>The exception:</p>
  
  <blockquote>
    <p>"The specified package is invalid. The main part is missing"</p>
  </blockquote>
  
  <p>What is the meaning of the exception and what is wrong?</p>
  
  <p>Thank in advance!</p>