Open the file with filename

def open_file(self, filename):
        """Open the file with filename"""
        try:
            if filename.endswith('.gz'):
                self.blob_file = gzip.open(filename, 'rb')
            else:
                self.blob_file = open(filename, 'rb')
        except TypeError:
            log.error("Please specify a valid filename.")
            raise SystemExit
        except IOError as error_message:
            log.error(error_message)
            raise SystemExit