code: public EmbedBuilder setThumbnail(BufferedImage thumbnail, String fileType) {
          delegate.setThumbnail(thumbnail, fileType);
          return this;
      }
docs: Sets the thumbnail of the embed.
  
  @param thumbnail The thumbnail.
  @param fileType The type of the file, e.g. "png" or "gif".
  @return The current instance in order to chain call methods.