Proxito: not setting cache tag on offline format files #12494
Description
Seems like we aren't setting cache tags on offline format files, so these files are never purged from the cache on purging.
% curl -ksI -H "Host: docs.readthedocs.com" https://web-1390472897.us-east-2.elb.amazonaws.com/platform/en/stable/ | grep -i cache-tag
cache-tag: docs,docs:en
% curl -ksI -H "Host: docs.readthedocs.com" https://web-1390472897.us-east-2.elb.amazonaws.com/_/downloads/platform/en/stable/epub/ | grep -i cache-tag
%
