code: public List<RequestFuture<?>> broadcast(byte[] data, String fileName)
      {
          Checks.notNull(data, "Data");
          Checks.check(data.length < Message.MAX_FILE_SIZE, "Provided data exceeds the maximum size of 8MB!");
          return broadcast(new WebhookMessageBuilder().addFile(fileName, data).build());
      }
docs: Sends the provided {@code byte[]} data as an attachment
  to all registered {@link net.dv8tion.jda.webhook.WebhookClient WebhookClients}.
  <br>Use {@link WebhookMessage#files(String, Object, Object...)} to send up to 10 files!
  
  <p><b>The provided data should not exceed 8MB in size!</b>
  
  @param  data
  The data that should be sent to the clients
  @param  fileName
  The name that should be given to the attachment
  
  @throws java.lang.IllegalArgumentException
  If the provided data is {@code null}
  @throws java.util.concurrent.RejectedExecutionException
  If any of the receivers has been shutdown
  
  @return A list of {@link java.util.concurrent.Future Future} instances
  representing all message tasks.