Mediawiki currently 400s where Thumbor serves the result:
Nov 15 11:20:37 ms-fe1001 proxy-server: HTTP status code mismatch. Mediawiki: 400 Thumbor: 200 URL: http://upload.wikimedia.org/wikipedia/en/thumb/c/cc/Windows_Media_Center_logo.png/64px-Windows_Media_Center_logo.png
Upscaling and same-size requests should 400 in Thumbor too.
The selected solution is store an extra X-Content-Dimensions header on originals in Swift with dimension information about the file. The header is to be populated on upload, and Thumbor will check it as early as possible to reject invalid requests.