allows /archive/main.tar.gz to work like /archive/main for compatibility with github-style archive URLs.
repro:
# before: returns 503 HTML
curl -sL "https://tangled.org/zzstoatzz.io/zat/archive/main.tar.gz" | file -
# after: should return gzip compressed data
curl -sL "https://tangled.org/zzstoatzz.io/zat/archive/main" | file -