replace mimetype with content_type

cartero/views.py

@@ -181,6 +181,6 @@ class MailTrackView(View):
         )
         openemail.save()
         img = Image.new("RGBA", (4, 4))
-        response = HttpResponse(mimetype="image/png")
+        response = HttpResponse(content_type="image/png")
         img.save(response, "PNG")
         return response