This happens on various thumbnail sizes of https://www.mediawiki.org/wiki/File:Page_Schemas_edit_schema_screenshot.png (not all of them).
Clicking on some of them, the browser prompts to download the file instead of displaying it. The cause is them being delivered with a application/x-www-form-urlencoded content-type instead of image/png
$ curl -s -D - -o /dev/null "https://upload.wikimedia.org/wikipedia/mediawiki/4/43/Page_Schemas_edit_schema_screenshot.png" HTTP/1.1 200 OK Date: Sat, 03 Mar 2018 17:15:19 GMT Content-Type: application/x-www-form-urlencoded Content-Length: 59510 Connection: keep-alive X-Object-Meta-Sha1Base36: 3lunouryyip6gpflvztb2p20l0nfs5k Last-Modified: Tue, 30 May 2017 11:07:47 GMT Etag: 5db3c2a1fee3897a8325c76b7d46ecd9 X-Timestamp: 1496142466.35998 X-Content-Dimensions: 1206x1349:1 X-Trans-Id: tx1ffbc88de8d04285bddbe-005a9ad71b X-Varnish: 721112451, 61187040 65427943, 466571330 Via: 1.1 varnish (Varnish/5.1), 1.1 varnish (Varnish/5.1), 1.1 varnish (Varnish/5.1) Accept-Ranges: bytes Age: 268 X-Cache: cp1073 pass, cp3034 hit/2, cp3039 miss X-Cache-Status: hit-local Strict-Transport-Security: max-age=106384710; includeSubDomains; preload X-Analytics: https=1;nocookies=1 X-Client-IP: 83.39.35.95 Access-Control-Allow-Origin: * Access-Control-Expose-Headers: Age, Date, Content-Length, Content-Range, X-Content-Duration, X-Cache, X-Varnish Timing-Allow-Origin: *