title: image embedded in email HTML template - Flask-mail ValueError: too many values to unpack (expected 2)
body: <p>I tried to send a response email from the HTML template that includes an image on the header.  If I send HTML without image attachment there is no error but it is a text email like. I searched and test for a couple of days, no success.</p>
  
  <p>Flask-email snippet is</p>
  
  <pre><code>msg = Message(form.title.data, recipients=['emial@gmail.com'])
  msg.html = render_template('mail_temp/file.html')
  with open('/home/daniel/PycharmProjects/Sitio/blue/static/pictures/ribbon.jpg', 'rb') as fp:
      msg.attach('ribbon.jpg', 'image/jpg', fp.read(), 'inline', headers={'Content-ID': 'ribbon'})
  mail.send(msg)
  </code></pre>
  
  <p>HTML template include image in:</p>
  
  <pre><code>&lt;td&gt; &lt;img src="cid:ribbon"/&gt; &lt;/td&gt;
  </code></pre>
  
  <p>The issue seems to happen in the parameter for Message. Attach method.  I found a similar question and response but 2 years old, maybe the method changes it.</p>
  
  <p>Thanks in advance. </p>
  
  <pre><code>  File "/home/daniel/anaconda3/envs/Sitio/lib/python3.8/site-packages/flask/app.py", line 1950, in full_dispatch_request
      rv = self.dispatch_request()
    File "/home/daniel/anaconda3/envs/Sitio/lib/python3.8/site-packages/flask/app.py", line 1936, in dispatch_request
      return self.view_functions[rule.endpoint](**req.view_args)
    File "/home/daniel/PycharmProjects/Sitio/blue/site/routes.py", line 220, in contactme
      mail.send(msg)
    File "/home/daniel/anaconda3/envs/Sitio/lib/python3.8/site-packages/flask_mail.py", line 492, in send
      message.send(connection)
    File "/home/daniel/anaconda3/envs/Sitio/lib/python3.8/site-packages/flask_mail.py", line 427, in send
      connection.send(self)
    File "/home/daniel/anaconda3/envs/Sitio/lib/python3.8/site-packages/flask_mail.py", line 190, in send
      message.as_bytes() if PY3 else message.as_string(),
    File "/home/daniel/anaconda3/envs/Sitio/lib/python3.8/site-packages/flask_mail.py", line 385, in as_bytes
      return self._message().as_bytes()
    File "/home/daniel/anaconda3/envs/Sitio/lib/python3.8/site-packages/flask_mail.py", line 371, in _message
      for key, value in attachment.headers:
  ValueError: too many values to unpack (expected 2)
  ``
  
  </code></pre>