Download response bodies from the playground
The result panel of the API playground has a Download button for non-empty responses. The file is named from the Content-Disposition header, preferring filename* and without directories, otherwise response, which browsers complete with the extension of the media type.
For the browser to read Content-Disposition, cross-origin APIs have to list it in Access-Control-Expose-Headers, or be requested through the proxy.
Fix #3625