code: protected OptionalThing<Object> prepareDynamicData(Postcard postcard, String bodyFile, boolean filesystem,
              OptionalThing<Locale> receiverLocale) {
          if (dynamicTextAssist == null) {
              return OptionalThing.empty();
          }
          final SMailDynamicDataResource resource = new SMailDynamicDataResource(postcard, bodyFile, filesystem, receiverLocale);
          @SuppressWarnings("unchecked")
          final OptionalThing<Object> dynamicData = (OptionalThing<Object>) dynamicTextAssist.prepareDynamicData(resource);
          if (dynamicData == null) {
              String msg = "Cannot return null as optional type: dynamicTextAssist=" + dynamicTextAssist + ", " + postcard;
              throw new SMailIllegalStateException(msg);
          }
          return dynamicData;
      }
docs: ------------