title: Exception using POI
body: <p>I'm using poi 3.7 , upload the file is .xlsx</p>
  
  <p>The console show:</p>
  
  <pre><code>org.apache.poi.POIXMLException: org.apache.poi.openxml4j.exceptions.InvalidFormatException: Package should contain a content type part [M1.13]
      at org.apache.poi.util.PackageHelper.open(PackageHelper.java:41)
      at org.apache.poi.xssf.usermodel.XSSFWorkbook.&lt;init&gt;(XSSFWorkbook.java:186)
      at poi.POITest.ReadAndPrintExcelFile(POITest.java:15)
      at poi.POITest.main(POITest.java:59)
  Caused by: org.apache.poi.openxml4j.exceptions.InvalidFormatException: Package should contain a content type part [M1.13]
      at org.apache.poi.openxml4j.opc.ZipPackage.getPartsImpl(ZipPackage.java:147)
      at org.apache.poi.openxml4j.opc.OPCPackage.getParts(OPCPackage.java:592)
      at org.apache.poi.openxml4j.opc.OPCPackage.open(OPCPackage.java:222)
      at org.apache.poi.util.PackageHelper.open(PackageHelper.java:39)
      ... 3 more
  </code></pre>