code: @Override
      public FileModel resolvePayload(GraphRewrite event, EvaluationContext context, WindupVertexFrame payload)
      {
          checkVariableName(event, context);
          if (payload instanceof FileReferenceModel)
          {
              return ((FileReferenceModel) payload).getFile();
          }
          if (payload instanceof FileModel)
          {
              return (FileModel) payload;
          }
          return null;
      }
docs: Set the payload to the fileModel of the given instance even though the variable is not directly referencing it. This is mainly to simplify the
  creation of the rule, when the FileModel itself is not being iterated but just a model referencing it.