support send attachment is instance of MINEBase

[x007007007]

According to django document, attribution attachments can be MIMEBase, if don't support that features, I don't know how to send inline attachment. I fix that, It look like work well.

[pydanny]

@x007007007 Thanks for the pull request! My one question is how exactly is this used? Let me know and I'll accept this and document what you say.

[vstoykov]

As what I see you can pass attachment to be different than tuple or list, and more precisely to be based of email.mime.base.MIMEBase. This probably is good if something else was constructed the attachment for you (some lib or whatever).

My suggestion about this PR is that the check for MIMEBase need to be first and to fallback to directly try to get 0 and 1 element instead of checking for if it is a list or tuple.

[x007007007]

you are right