Switched mimetype to content_type because of deprecation

The mimetype keyword was deprecated in Django 1.5,
https://docs.djangoproject.com/en/1.5/ref/request-response/#django.http.HttpResponse.__init__

The content_type keyword should be used instead.
Merge pull request #1488 from binarydud/mimetype_deprecated

Switched mimetype to content_type because of deprecation