code: def check_file(self):
          
          try:
              image_type = ".{0}".format(self.image.split(".")[1])
              if image_type not in self.file_format:
                  print("Format: '{0}' not recognized. Use one of "
                        "them:\n{1}".format(self.image.split(".")[1],
                                            ", ".join(self.file_format)))
                  raise SystemExit()
          except IndexError:
              print("slpkg: Error: Image file suffix missing")
              raise SystemExit()
docs: Check for file format and type