According to the documentation, one should be able to set objects headers on upload to GoogleCloudStorage.
Implementation Details
You should specify cache-control only for objects that are accessible to all anonymous users. To be anonymously accessible, an object's ACL must grant READ or FULL_CONTROL permission to AllUsers. If an object is accessible to all anonymous users and you do not specify a cache-control setting, Cloud Storage applies a cache-control setting of 3600 seconds. When serving via XML, Cloud Storage respects the cache-control of the object as set by its metadata.
However, adding headers through the Google API doesn't seem to work, when fetching the image back with google.appengine.api.images.get_serving_url .
Changing Cache-Control headers from gsutil console has its effects, but takes several days for changes to be visible on the object (when checking from the gsutil console, again, no effect when fetching the image back with the API.
After 2 months of going back and forth with Google's support, we found out that file is sent to the Google Cloud Storage with the proper headers (can be checked via gsutil command).
However get_serving_url function does not respect Blob's headers (confirmed by Google's engineers).
As of 17th of August 2017, there are no future plans to fix that. Thought someone may encounter the similar problem as there's nothing about it in the documentation.