Set `expires` by MIME type

[petecooper]

Hello.
I am attempting to use `expires` on Nginx 1.15.3 to define the expiry of
files on a per MIME type basis.

I have used [1] as a base, and constructed the following `map` in the `http`
section of a `include`-d `server` block (domain sanitised):

    map $sent_http_content_type $www_example_com_expires {
        default 1M;
        application/atom+xml 1h;
        application/javascript 1M;
        application/json 0s;
        application/ld+json 0s;
        application/manifest+json 1w;
        application/rdf+xml 1h;
        application/rss+xml 1h;
        application/schema+json 0s;
        application/x-javascript 1M;
        application/xml 0s;
        font/woff 1M;
        image/gif 1M;
        image/jpeg 1M;
        image/png 1M;
        image/svg+xml 1M;
        image/vnd.microsoft.icon 1M;
        image/webp 1M;
        image/x-icon 1M;
        text/cache-manifest 0s;
        text/css 1M;
        text/html 0s;
        text/javascript 1M;
        text/x-cross-domain-policy 1w;
        text/xml 0s;
        video/mp4 1M;
        video/webm 1M;
    }

Later on, after the `map` is defined, I call it using `expires` in a
`server` block:

    server {#IPv4 and IPv6, https, PHP fastcgi, check https://cipherli.st
for latest ciphers
        access_log /var/log/nginx/www.example.com.access.log ipscrubbed;
        add_header Access-Control-Allow-Origin "https://*.example.com";
        add_header Content-Security-Policy "default-src 'self'; connect-src
'self' https://api.github.com; font-src 'self'; img-src 'self' data: *
https://*; media-src 'self' * https://*; style-src 'self' 'unsafe-in$
        add_header Expect-CT "max-age=0;
report-uri=https://example.com/expect-ct-report";
        add_header Feature-Policy "camera 'self'; geolocation 'none';
microphone 'none'; midi 'none'; payment 'none'";
        add_header Referrer-Policy strict-origin;
        add_header Strict-Transport-Security "max-age=15768000;
includeSubDomains; preload";
        add_header X-Content-Type-Options nosniff;
        add_header X-Frame-Options DENY;
        add_header X-XSS-Protection "1; mode=block";
        error_log /var/log/nginx/www.example.com.error.log crit;
        etag off;
        expires $www_example_com_expires;
        index index.html index.php;
        listen [::]:443 http2 ssl;
        listen 443 http2 ssl;
        [...]
    }

My config passes the `nginx -t` self-test with no errors, and I can restart
Nginx without issue.

In the browser inspector, all MIME types are assigned a 1 month expiry, as
if they're inheriting the `default` value from the map. Example headers for
a .php file:

    Date: Mon, 03 Sep 2018 20:09:30 GMT
    Expires: Wed, 03 Oct 2018 20:09:30 GMT

If I remove the `expires` directive, the 'Expires:' header is not shown, so
`expires` is doing *something*.

I suspect my syntax is wrong, and I would be very grateful for any feedback
-- I am particularly interested a clue or pointer to aid my research into
why this is not working.

Thank you for your attention and interest.

[1] http://nginx.org/en/docs/http/ngx_http_headers_module.html#expires

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,281087,281087#msg-281087