docs: private void setAttribute( DBCollection content,
                                 String fieldName,
                                 Object value ) {
          DBObject header = content.findOne(HEADER_QUERY);
          BasicDBObject newHeader = new BasicDBObject();
  
          // clone header
          newHeader.put(FIELD_CHUNK_TYPE, header.get(FIELD_CHUNK_TYPE));
          newHeader.put(FIELD_MIME_TYPE, header.get(FIELD_MIME_TYPE));
          newHeader.put(FIELD_EXTRACTED_TEXT, header.get(FIELD_EXTRACTED_TEXT));
          newHeader.put(FIELD_UNUSED, header.get(FIELD_UNUSED));
          newHeader.put(FIELD_UNUSED_SINCE, header.get(FIELD_UNUSED_SINCE));
  
          // modify specified field and update record
          newHeader.put(fieldName, value);
          content.update(HEADER_QUERY, newHeader);
      }