In my GAE app I am serving static content as follows (those are my entries in my app.yaml file):
handlers:
- url: /c
After the hint from Martijn above I changed expiration values in my app.yaml to:
handlers:
- url: /css
static_dir: static/css
expiration: "0d 10m"
- url: /js
static_dir: static/js
expiration: "0d 10m"
Now everything works as expected and the I get the following headers in response:
cache-control:public, max-age=600
content-encoding:gzip
Everything seems to work as expected now.