upload.wikimedia.org is the hostname used for access to original and scaled media for the Wikimedia projects.
For more on the current media server architecture, see Media storage.
Our private wikis do not serve their files to the public this way; they're served through img_auth.php on the local domain, which enforces authentication.
Most of our wikis have a rewrite rule to redirect requests from /upload and /math on the primary wiki domain to the appropriate subdirectory on upload.wikimedia.org. This provides compatibility for old direct image links from the days before the separate image hosting.
Special:Filepath can also send redirects here.