title: smtp send email and why one attachment can have two Content-Type?
body: <p>I'm trying use smtp to send email with attachment.And when I get raw email,there are two content-type for one attachment.
  How can I just get one content-type?And the two type impact each other?
  Thanks for any help!</p>
  
  <pre><code>import smtplib
  from email.mime.multipart import MIMEMultipart
  from email.mime.application import MIMEApplication
  
  server = smtplib.SMTP()
  server.connect("smtp.XX.com")
  
  server.login("","")
  
  msg = MIMEMultipart("")
  msg['From'] = ""
  msg['Subject'] = "titlesub"
  part = MIMEApplication(open("D:\data.txt", 'rb').read())
  filename="data.txt"
  #part['Content-Type']="application/pdf"
  part.add_header('Content-Type','application/pdf')
  part.add_header('Content-Disposition', 'attachment', filename=filename)
  
  msg.attach(part)
  msg['To'] = ""
  server.send_message(msg)
  server.quit()
  </code></pre>
  
  <p>Raw email:</p>
  
  <pre><code>Received: from [127.0.0.1] (unknown [101.81.225.242])
  by smtp8 (Coremail) with SMTP id DMCowABH3zUeOgBZsU+uAg--.2242S2;
  Wed, 26 Apr 2017 14:11:42 +0800 (CST)
  Content-Type: multipart/; boundary="===============4516509904929376112=="
  MIME-Version: 1.0
  From: 
  Subject: titlesub
  To: 
  X-CM-TRANSID:DMCowABH3zUeOgBZsU+uAg--.2242S2
  Message-Id:&lt;59003A1E.C4DB82.14752@m12-12.163.com&gt;
  X-Coremail-Antispam: 1Uf129KBjDUn29KB7ZKAUJUUUUU529EdanIXcx71UUUUU7v73
  VFW2AGmfu7bjvjm3AaLaJ3UbIYCTnIWIevJa73UjIFyTuYvjxUkebkUUUUU
  X-Originating-IP: [101.81.225.242]
  Date: Wed, 26 Apr 2017 14:11:42 +0800 (CST)
  X-CM-SenderInfo: pix130tbbsiiqu6rljoofrz/1tbivh7F0FZcM5OV1wAAsd
  
  --===============4516509904929376112==
  Content-Type: application/octet-stream
  MIME-Version: 1.0
  Content-Transfer-Encoding: base64
  Content-Type: application/pdf
  Content-Disposition: attachment; filename="data.txt"
  
  77u/
  
  --===============4516509904929376112==--
  </code></pre>