This restriction is a security hack for some (older?) browsers which can be fooled into executing arbitrary HTML+JavaScript code when there's a "recognized file extension" at the end of the URL path.

To avoid hitting this, either:

• use the full index.php URL: https://en.wikipedia.org/w/index.php?title=Washington_University_in_St._Louis&action=raw

or

• use the API to fetch data in XML or JSON packaging

Note that action=raw is a very old feature and is not recommended for current use since the API was created some years ago.