如何使用 SMTP 发送附件？

我想编写一个使用Python发送电子邮件的程序smtplib http://docs.python.org/library/smtplib.html。我搜索了文档和 RFC，但找不到任何与附件相关的内容。因此，我确信我错过了一些更高层次的概念。有人可以告诉我附件在 SMTP 中是如何工作的吗？

以下是包含 PDF 附件、文本“正文”并通过 Gmail 发送的邮件示例。

# Import smtplib for the actual sending function
import smtplib

# For guessing MIME type
import mimetypes

# Import the email modules we'll need
import email
import email.mime.application

# Create a text/plain message
msg = email.mime.Multipart.MIMEMultipart()
msg['Subject'] = 'Greetings'
msg['From'] = '[email protected] /cdn-cgi/l/email-protection'
msg['To'] = '[email protected] /cdn-cgi/l/email-protection'

# The main body is just another attachment
body = email.mime.Text.MIMEText("""Hello, how are you? I am fine.
This is a rather nice letter, don't you think?""")
msg.attach(body)

# PDF attachment
filename='simple-table.pdf'
fp=open(filename,'rb')
att = email.mime.application.MIMEApplication(fp.read(),_subtype="pdf")
fp.close()
att.add_header('Content-Disposition','attachment',filename=filename)
msg.attach(att)

# send via Gmail server
# NOTE: my ISP, Centurylink, seems to be automatically rewriting
# port 25 packets to be port 587 and it is trashing port 587 packets.
# So, I use the default port 25, but I authenticate. 
s = smtplib.SMTP('smtp.gmail.com')
s.starttls()
s.login('[email protected] /cdn-cgi/l/email-protection','xyzpassword')
s.sendmail('[email protected] /cdn-cgi/l/email-protection',['[email protected] /cdn-cgi/l/email-protection'], msg.as_string())
s.quit()