docs: def all_sample_md5s(self, type_tag=None):
          """Return a list of all md5 matching the type_tag ('exe','pdf', etc).
  
          Args:
              type_tag: the type of sample.
  
          Returns:
              a list of matching samples.
          """
  
          if type_tag:
              cursor = self.database[self.sample_collection].find({'type_tag': type_tag}, {'md5': 1, '_id': 0})
          else:
              cursor = self.database[self.sample_collection].find({}, {'md5': 1, '_id': 0})
          return [match.values()[0] for match in cursor]