guess attachment mimetype in Django email backend if not provided

[HugoArts]

Hello,

This is an issue I ran into while using django-post-office. The django EmailMessage.attach method does not require you to submit a mimetype, stating that it is guessed if not provided. Unfortunately the django-post-office API does not support specifying a mime type, meaning I have no choice but to rely on the guessing mechanism. Long story short, I cannot currently attach any files to my emails.

The email backend would normally let the EmailMessage class do this when converting the email to MIME format (see the EmailMessage.message method which eventually calls EmailMessage._create_attachment). Obviously the sparkpost backend does not call any of this code.

I've duplicated django's method of guessing the mimetype into the sparkpost backend. I've also added two tests to demonstrate the problem.

[richleland]

Thanks for the contribution @HugoArts. I'm going to get #50 merged in and then cut a release.