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I don't really understand what you mean by "email multipart message object". Do you mean an object belonging to the email.message.Message class?

If that is what you mean, it's straightforward. On a multipart message, the get_payload method returns a list of message parts (each of which is itself a Message object). You can iterate over these parts and examine their properties: for example, the get_content_type method returns the part's MIME type, and the get_filename method returns the part's filename (if any is specified in the message). Then when you've found the correct message part, you can call get_payload(decode=True) to get the decoded contents.

>>> import email
>>> msg = email.message_from_file(open('message.txt'))
>>> len(msg.get_payload())
>>> attachment = msg.get_payload()[1]
>>> attachment.get_content_type()
'image/png'
>>> open('attachment.png', 'wb').write(attachment.get_payload(decode=True))
If you're programmatically extracting attachments from email messages you have received, you might want to take precautions against viruses and trojans. In particular, you probably ought only to extract attachments whose MIME types you know are safe, and you probably want to pick your own filename, or at least sanitize the output of get_filename.
                HTML mails often have images in the footers, which are also sent as attachments. You can distinguish these from "real" attachments by looking at the Content-Disposition: inline images start with "inline", while actual attachments start with"attachment". There's no method for getting the content disposition, but you can call part.get('Content-Disposition').startswith('attachment') if you're only interested in actual attachments.
– jrial
                Jun 25, 2017 at 15:21
typ, data = self.imap.uid('SEARCH', 'ALL')
msgs = data[0].split()
print "Found {0} msgs".format(len(msgs))
for uid in msgs:
    typ, s = self.imap.uid('FETCH', uid, '(RFC822)')
    mail = email.message_from_string(s[0][1])
    print "From: {0}, Subject: {1}, Date: {2}\n".format(mail["From"], mail["Subject"], mail["Date"])
    if mail.is_multipart():
        print 'multipart'
        for part in mail.walk():
            ctype = part.get_content_type()
            if ctype in ['image/jpeg', 'image/png']:
                open(part.get_filename(), 'wb').write(part.get_payload(decode=True))
Actually using now-suggested email.EmailMessage API (don't confuse with old email.Message API) it is fairly easy to:
Iterate over all message elements and select only attachments  
Iterate over just attachments
Let's assume that you have your message stored as byte content in envelope variable 
Solution no.1:
import email
from email.message import EmailMessage
email_message: EmailMessage = email.message_from_bytes(envelope, _class=EmailMessage)
for email_message_part in email_message.walk():
    if email_message.is_attachment():
        # Do something with your attachment
Solution no.2: (preferable since you don't have to walk through other parts of your message object)
import email
from email.message import EmailMessage
email_message: EmailMessage = email.message_from_bytes(envelope, _class=EmailMessage)
for email_message_attachment in email_message.iter_attachments():
        # Do something with your attachment
Couple things to note:
We explicitly tell to use new EmailMessage class in our byte read method through _class=EmailMessage parameter
You can read your email message (aka envelope) from sources such as bytes-like object, binary file object or string thanks to built-in methods in message.Parser API 
        
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