code: public void setFile(File file, String attachmentFilename, String contentType) {
          this.file = file;
          this.attachmentFilename = attachmentFilename;
          this.contentType = contentType;
      }
docs: Sets the file to send to the client. If a file is set then the framework will try to read it
  and write it into the response using a FileSerializer.
  
  @param file The file.
  @param attachmentFilename Name for the file that will be sent.
  @param contentType Content type for the response header.