Send email with inline images asynchronously

[DenisBotev]

Emails sent using EmailMultiAlternatives are not sent through Celery, but through the standard Django backend. At least not in my case - perhaps I missed something. Either way, I found the following workaround to attach an inline image to mail.send which might be useful to others:

We initially follow the instructions in the README up until the initialization of the EmailMultiAlternatives.
Then, we instead create a dummy EmailMultiAlternatives instance and attach the image like in the tutorial:

dummy_multialt = EmailMultiAlternatives(subject, body, from_email, [to_email])
template = get_template('email-template-name.html', using='post_office')
context = {...}
html = template.render(context)
dummy_multialt.attach_alternative(html, 'text/html')
template.attach_related(dummy_multialt)

Then, we get the newly attached image using a somewhat ugly solution:

inline_image = template.template._attached_images[0]  # in case we attached just one image
inline_image_headers = dict(inline_image.__dict__["_headers"])  # cast image headers to a dict in order to plug them directly later
inline_image_cid = inline_image_headers["Content-ID"].replace("<", "").replace(">", "")

and finally:

mail.send(
    'recipient@example.com', 
    'from@example.com',
    subject='My email',
    message='Hi there!',
    html_message=html,
    attachments={
        inline_image_cid: {
            "file": ContentFile(inline_image.get_payload()),
            "mimetype": "image/png", 
            "headers": inline_image_headers
        }
    }
)

I'm sure there must be a cleaner way, but I'm not really experienced with MIME objects.
Currently I can't think of a way to include this painlessly into the codebase, this is why I'm posting it as an Issue.