SECURITY: API: Avoid some silliness with browser-guessed filenames

SECURITY: API: Avoid some silliness with browser-guessed filenames

If someone is both dumb enough to blindly save an API response and to
then execute the resulting file, this can be used to attack their
computer.

We can mitigate this by disallowing PATH_INFO in api.php URLs (because
we don't make any use of them anyway) and by setting a sensible filename
using a Content-Disposition header so the browser won't go guessing at
the filename based on what is in the URL.

Issue reported by: Abdullah Hussam

Bug: T128209
Change-Id: I8526f5cc506c551edb6138d68450b6acea065e93