another try at fix

misago/threads/models/attachment.py

@@ -1,9 +1,11 @@
+from io import BytesIO
+
 from django.conf import settings
 from django.core.files import File
 from django.core.files.base import ContentFile
 from django.core.urlresolvers import reverse
 from django.db import models
-from django.utils import six, timezone
+from django.utils import timezone
 from django.utils.crypto import get_random_string
 from PIL import Image
 
@@ -79,12 +81,9 @@ class Attachment(models.Model):
         thumbnail = Image.open(upload)
         thumbnail.thumbnail((500, 500))
 
-        if six.PY3:
-            thumb_stream = six.StringIO()
-        else:
-            thumb_stream = six.BytesIO()
+        thumb_stream = BytesIO()
         thumbnail.save(thumb_stream, fileformat)
 
         thumb_secret = get_random_string(settings.MISAGO_ATTACHMENT_SECRET_LENGTH)
         thumb_filename = '.'.join([thumb_secret, fileformat])
-        self.thumbnail = ContentFile(thumb_stream, thumb_filename)
+        self.thumbnail = ContentFile(thumb_stream.get_value(), thumb_filename)