code: public String createContainingBase64Value(String basename, String key) {
          String file;
          Object value = value(key);
          if (value == null) {
              throw new SlimFixtureException(false, "No value for key: " + key);
          } else if (value instanceof String) {
              file = createFileFromBase64(basename, (String) value);
          } else {
              throw new SlimFixtureException(false, "Value for key: " + key + " is not a String, but: " + value);
          }
          return file;
      }
docs: Saves content of a key's value as file in the files section.
  @param basename filename to use.
  @param key key to get value from.
  @return file created.