upload.wikimedia.org is the hostname used for access to original and scaled-down media files for the Wikimedia projects.
The MediaWiki software is configured at WMF to read-write user generated media files to Swift backends (see wmf-config/filebackend)). Each wiki has its own Swift bucket, and these are exposed through an Nginx proxy at upload.wikimedia.org. For example, files from English Wikipedia (en.wikipedia.org) are served from
On private wikis, media files are not served from "upload.wikimedia.org", but instead served through the
img_auth.php MediaWiki entry point from the private wiki's own domain name (which enforces authentication and access control).