title: sendPhoto() in Telepot doesn't work for my bot
body: <p>I can't send photo, here the code with error:</p>
  
  <pre><code>if command.startswith('/rank '):
      rank(msg)
  
  def rank(msg):
      chat_id = msg['chat']['id']
      command = msg['text']
  
      user = msg['text'][6:]
      graphUrl = 'https://www.website.com/servlet/graph/' + user + '-in_US.png'
      print graphUrl
  
      theGraph = urllib2.urlopen(graphUrl)
  
      bot.sendPhoto(chat_id, theGraph, caption=('rank graph for ' + user + '.'))
  </code></pre>
  
  <blockquote>
    <p>Error: 2016-12-30T17:17:50.803142+00:00 app[worker.1]: TelegramError:
    (u'Bad Request: Photo has unsupported extension. Use one of .jpg,
    .jpeg, # .gif, .png, .tif or .bmp', 400, {u'ok': False,
    u'description': u'Bad Request: Photo has unsupported extension. Use
    one of .jpg, .jpeg, # .gif, .png, .tif or .bmp', u'error_code': 400})</p>
  </blockquote>
  
  <p>My file is <code>.png</code>, where am I wrong?
  If I replace <code>sendPhoto()</code> with <code>sendDocument()</code> everything works perfectly, but I need photos in my project.
  If I put directly <code>graphUrl</code> in sendPhoto, without using urllib2, it doesn't work (error 400 - bad request).</p>