Thumbor request URL
curl -v 'http://...../unsafe/fit-in/400x/filters:format(png)/.......' >out.png
Any PNG with transparency should work. Here is a test image:

Expected behaviour
The background of out.png should be transparent.
Actual behaviour
The background of out.png is black.
Operating system
Debian Linux
Your thumbor.conf
thumbor.conf.txt
Scaling the image seems to be causing it. When I specify a width larger than the original, transparency is not lost.