[3.10] gh-68966: Make mailcap refuse to match unsafe filenames/types/params (GH-91993) (GH-93543)

* gh-68966: Make mailcap refuse to match unsafe filenames/types/params (GH-91993)
(cherry picked from commit b9509ba7a9c668b984dab876c7926fe1dc5aa0ba)
* Add a What's New entry for 3.10.8.

Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>