code: public static String getSuffix(String mimeType) {
  	if (mimeType.contains("openxmlformats-officedocument.spreadsheetml")) {
  		return ExcelFileOutputFormat.SUFFIX_OOXML;
  	} else if (mimeType.contains("ms-excel")) {
  		return ExcelFileOutputFormat.SUFFIX_OLDEXCEL;
  	} 
  	return ".unknown";
  }
docs: /*
  Determines file extension based on MimeType
  
  @param mimeType mimeType of the file
  
  @return file extension