MIME Multipart being sent as "noname" on Python 3

Question

I had a script made on Python 2.7 that would send an email. The following code worked perfectly on it:

msg = MIMEMultipart(
    From = "Name <mail@from.com>",
    To = "Name <mail@to.com>",
    Date=formatdate(localtime=True),
    Subject="SUBJECT")
msg.attach(MIMEText("""HTML Stuff""", 'html'))

with open(analysis_file, "rb") as fil:
    msg.attach(MIMEApplication(
                fil.read(),
                Content_Disposition='attachment; filename="%s"' % os.path.basename(analysis_file),
                Name=os.path.basename(analysis_file)
            ))

msg['Subject'] = "SUBJECT"
msg['From'] = "Name <mail@from.com>"
msg['To'] = "Name <mail@to.com>"

server.sendmail(FROMADDR, ["mail@to.com"], msg.as_string())

I converted the script to Python 3.5, and all the other parts of the script work exactly as expected. The above, however, does not: instead of sending my email with the HTML Stuff and the attachment, it sends a blank email with an extensionless file called "noname" which I presume has the stuff I wanted to send.

I tried removing the attachment part and keeping only the HTML stuff, but still no deal.

Why does this happen, and how can I fix it?

(The reason I put Subject, From, and To twice is that when I used it only the first time it wouldn't work for some reason, and I never removed it from there after it started working.)

Answer 1

I figured it out by rerereading the email documentation and replacing

msg = MIMEMultipart(
    From = "Name <mail@from.com>",
    To = "Name <mail@to.com>",
    Date=formatdate(localtime=True),
    Subject="SUBJECT")

with

msg = MIMEMultipart('alternative')