Treat image attachments with a Content-ID header as embedded, rather than ordinary attachments. (Rationale is that you must set the content-id to be able to refer to the image within your html, so that's a reasonable indicator to handle it as embedded.)
Introduce djrill.NotSupportedByMandrillError for unsupported functionality (previously used generic ValueError).
Change to djrill.MandrillAPIError, derived from requests.HTTPError, for API error responses (previously used djrill.mail.backends.djrill.DjrillBackendHTTPError -- retained as equivalent for backwards compatibility).
Add DjrillBackendMockAPITestCase.assert_mandrill_called; use it in representative backend test cases.
(Also make get_api_call_data work with various ways of calling requests.post.)