code: public static String getFileFormat(POIFSFileSystem fs) throws IOException
     {
        String fileFormat = "";
        DirectoryEntry root = fs.getRoot();
        if (root.getEntryNames().contains("\1CompObj"))
        {
           CompObj compObj = new CompObj(new DocumentInputStream((DocumentEntry) root.getEntry("\1CompObj")));
           fileFormat = compObj.getFileFormat();
        }
        return fileFormat;
     }
docs: This method allows us to peek into the OLE compound document to extract the file format.
  This allows the UniversalProjectReader to determine if this is an MPP file, or if
  it is another type of OLE compound document.
  
  @param fs POIFSFileSystem instance
  @return file format name
  @throws IOException