To send an e-mail with mixed content you requires to set Content-type header to multipart/mixed. Then, text and attachment sections can be specified within boundaries.
A boundary started with two hyphens followed by a unique number, which cannot appear in the message part of the e-mail. A final boundary denoting the e-mail's final section must also end with two hyphens. For example you can see below code.

!/shiva/bin/python

import smtplib
import base64


filename = "/tmp/test.txt"

Read a file and encode it into base64 format

fo = open(filename, "rb")
filecontent = fo.read()
encodedcontent = base64.b64encode(filecontent)  # base64

sender = 'webmaster@tutorialpoint.com'
reciever = 'amrood.admin@gmail.com'

marker = "AUNIQUEMARKER"

body ="""
This is a test email to send an attachement.
"""

Define the main headers.

part1 = """From: From Person me@fromdomain.net
To: To Person amrood.admin@gmail.com
Subject: Sending Attachement
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary=%s
--%s
""" % (marker, marker)


Define the message action

part2 = """Content-Type: text/plain
Content-Transfer-Encoding:8bit


%s
--%s
""" % (body,marker)

Define the attachment section
part3 = """Content-Type: multipart/mixed; name=\"%s\"
Content-Transfer-Encoding:base64
Content-Disposition: attachment; filename=%s
%s
--%s--
""" %(filename, filename, encodedcontent, marker)
message = part1 + part2 + part3

try:
   smtpObj = smtplib.SMTP('localhost')
   smtpObj.sendmail(sender, reciever, message)
   print "Successfully sent email"
except Exception:
   print "Error: unable to send email"