title: Telegram sending png picture to Telegram converts it to jpg
body: <p>Why is that when I send a png picture to telegram bot, and then download it, it becomes jpg? How to avoid it?</p>
  
  <pre><code>MY_USER_ID = 012345
  MY_TOKEN = "12345"
  import logging
  logging.basicConfig(format='%(asctime)s - %(name)s - %(levelname)s - %(message)s', level=logging.INFO)
  
  from telegram.ext import Updater, MessageHandler, Filters, CommandHandler
  
  updater = Updater(token=MY_TOKEN, use_context=True)
  
  updater.start_polling()
  
  dispatcher = updater.dispatcher
  
  
  def create_sticker_set(update, context):
  
      update_dict = update.to_dict()
  
      sticker_file_id = update_dict["message"]["photo"][-1]["file_id"]
  
      print("sticker_file_id=", sticker_file_id)
  
      file = context.bot.get_file(update_dict["message"]["photo"][-1]["file_id"])
      filename = file.download() # it is jpg, and it must be png
  
      context.bot.add_sticker_to_set(MY_USER_ID, "lala_by_ibodi_bot", open(filename, "rb"), "")
  
  
  msghandler = MessageHandler(Filters.photo, create_sticker_set)
  dispatcher.add_handler(msghandler)
  </code></pre>
  
  <p>This code creates a handler that receives a picture and adds it to a sticker set. I want to preserve the extension of the picture so that transparent parts of the picture stayed transparent in a sticker set. And converting it to jpg makes transparent places white.</p>