title: How to send Image to Slack as slack bot via slackclient?
body: <p>I'm building a simple bot to periodically generate charts as images and post to slack via the python <a href="https://github.com/slackapi/python-slackclient" rel="nofollow noreferrer">slackclient</a> package.</p>
  
  <p>I've managed to send a bot message with the code below:</p>
  
  <pre><code>def post_message_to_channel(self, channel_id, message=DEFAULT_TEST_MESSAGE):
      sc = SlackClient(TOKEN)
      sc.api_call(
          "chat.postMessage",
          channel=channel_id,
          username='mybot',
          text='test text message'
      )
  </code></pre>
  
  <p>But when I try to do the same with a file upload, the image is sent correctly, but it shows my name, and not the specified bot name:</p>
  
  <pre><code>def post_image_to_channel(self, channel_name, filepath, tile='Test Upload'):
      sc = SlackClient(TOKEN)
      with open(filepath, 'rb') as file_content:
          sc.api_call(
                  "files.upload",
                  channels=channel_id,
                  file=file_content,
                  title=tile,
                  username='mybot',               
              )
  </code></pre>
  
  <p>Looking at the slack API documentation for <a href="https://api.slack.com/methods/files.upload" rel="nofollow noreferrer">files.upload,</a> it appears that the <code>username</code> is not available.</p>
  
  <p>How can I send the <em>files.upload</em> image using the bot name?</p>