html_content with attachment in django-post_office

I get strange bug with attachment in django-post-office (at least, it's encountered in version 2.0.8 and lower): when the mail is sent, which consist of html-content with attachment (of any format), the mail is come, but without attachment.

I used next package version:

django==1.10.7 (and lower)
django-post-office==2.0.8 (and lower)

In more detail, the behavior of django-post-office of the specified version and the working of the mail.send() method can be described as follows:

1. When the mail is sent with simple content, using message parameter, and with attachment - it works correctly, i. e. I get mail with attachment.
2. When the mail is sent with content, which contains html-tags or even plain text without tags, using html_message parameter, and with attachment - it does not work correctly, i. e. I get the empty mail, but file is attached.
3. When the mail is sent with content, using html_message parameter, but without attachment - it work correctly, i. e. I get the mail formated by html-tags and with attachment.

And I find the workaround of this strange gug: use both arguments message and html_message. For example:

msg = render_to_string('path/to/template/', {'some_data': 'some_data'})
pdf = render_to_pdf(form.pdf_template, email_data)
attachments = {'Message.pdf': ContentFile(pdf)}
mail.send(user_email, me, subject=subject, message=msg, html_message=msg, attachments=attachments)