Here is the show case.
Works properly most of the time, for example here:
$ curl -I https://upload.wikimedia.org/wikipedia/commons/thumb/1/1c/De_Rotterdam%2C_September_2019_-_02.jpg/360px-De_Rotterdam%2C_September_2019_-_02.jpg HTTP/2 200 date: Wed, 07 Oct 2020 21:14:44 GMT content-type: image/jpeg content-length: 70536 last-modified: Wed, 11 Sep 2019 03:32:21 GMT etag: 6ea8427bb3c0df7b880989b9022f5428 x-timestamp: 1568172740.52202 server: ATS/8.0.8 age: 47424 x-cache: cp3055 hit, cp3055 hit/9 x-cache-status: hit-front server-timing: cache;desc="hit-front" strict-transport-security: max-age=106384710; includeSubDomains; preload report-to: { "group": "wm_nel", "max_age": 86400, "endpoints": [{ "url": "https://intake-logging.wikimedia.org/v1/events?stream=w3c.reportingapi.network_error&schema_uri=/w3c/reportingapi/network_error/1.0.0" }] } nel: { "report_to": "wm_nel", "max_age": 86400, "failure_fraction": 0.05, "success_fraction": 0.0} x-client-ip: 2a02:168:6008:0:e998:bc9d:2fa5:b439 access-control-allow-origin: * access-control-expose-headers: Age, Date, Content-Length, Content-Range, X-Content-Duration, X-Cache timing-allow-origin: * accept-ranges: bytes $ curl --header 'If-None-Match: 6ea8427bb3c0df7b880989b9022f5428' -I https://upload.wikimedia.org/wikipedia/commons/thumb/1/1c/De_Rotterdam%2C_September_2019_-_02.jpg/360px-De_Rotterdam%2C_September_2019_-_02.jpg HTTP/2 304 date: Wed, 07 Oct 2020 21:14:44 GMT content-type: image/jpeg last-modified: Wed, 11 Sep 2019 03:32:21 GMT etag: 6ea8427bb3c0df7b880989b9022f5428 x-timestamp: 1568172740.52202 server: ATS/8.0.8 age: 47433 x-cache: cp3055 hit, cp3055 hit/10 x-cache-status: hit-front server-timing: cache;desc="hit-front" strict-transport-security: max-age=106384710; includeSubDomains; preload report-to: { "group": "wm_nel", "max_age": 86400, "endpoints": [{ "url": "https://intake-logging.wikimedia.org/v1/events?stream=w3c.reportingapi.network_error&schema_uri=/w3c/reportingapi/network_error/1.0.0" }] } nel: { "report_to": "wm_nel", "max_age": 86400, "failure_fraction": 0.05, "success_fraction": 0.0} x-client-ip: 2a02:168:6008:0:e998:bc9d:2fa5:b439 access-control-allow-origin: * access-control-expose-headers: Age, Date, Content-Length, Content-Range, X-Content-Duration, X-Cache timing-allow-origin: *
But for this specific image/request the HTTP response header content-type is missing:
$ curl -I https://upload.wikimedia.org/wikipedia/commons/thumb/9/94/CIA_map_of_Central_America.png/490px-CIA_map_of_Central_America.png HTTP/2 200 date: Thu, 08 Oct 2020 08:02:57 GMT content-type: image/png content-length: 328121 accept-ranges: bytes last-modified: Sun, 20 May 2018 15:52:08 GMT etag: 122f752850864671485c9d915f0feca2 x-timestamp: 1526831527.13817 server: ATS/8.0.8 age: 0 x-cache: cp3055 hit, cp3055 pass x-cache-status: hit-local server-timing: cache;desc="hit-local" strict-transport-security: max-age=106384710; includeSubDomains; preload report-to: { "group": "wm_nel", "max_age": 86400, "endpoints": [{ "url": "https://intake-logging.wikimedia.org/v1/events?stream=w3c.reportingapi.network_error&schema_uri=/w3c/reportingapi/network_error/1.0.0" }] } nel: { "report_to": "wm_nel", "max_age": 86400, "failure_fraction": 0.05, "success_fraction": 0.0} x-client-ip: 2a02:168:6008:0:e998:bc9d:2fa5:b439 access-control-allow-origin: * access-control-expose-headers: Age, Date, Content-Length, Content-Range, X-Content-Duration, X-Cache timing-allow-origin: * $ curl --header 'If-None-Match: 122f752850864671485c9d915f0feca2' -I https://upload.wikimedia.org/wikipedia/commons/thumb/9/94/CIA_map_of_Central_America.png/490px-CIA_map_of_Central_America.png HTTP/2 304 date: Thu, 08 Oct 2020 10:26:20 GMT etag: 122f752850864671485c9d915f0feca2 server: ATS/8.0.8 age: 0 x-cache: cp3055 hit, cp3055 pass x-cache-status: hit-local server-timing: cache;desc="hit-local" strict-transport-security: max-age=106384710; includeSubDomains; preload report-to: { "group": "wm_nel", "max_age": 86400, "endpoints": [{ "url": "https://intake-logging.wikimedia.org/v1/events?stream=w3c.reportingapi.network_error&schema_uri=/w3c/reportingapi/network_error/1.0.0" }] } nel: { "report_to": "wm_nel", "max_age": 86400, "failure_fraction": 0.05, "success_fraction": 0.0} x-client-ip: 2a02:168:6008:0:e998:bc9d:2fa5:b439 access-control-allow-origin: * access-control-expose-headers: Age, Date, Content-Length, Content-Range, X-Content-Duration, X-Cache timing-allow-origin: *