[Validator] File: add option to check extension

[fabpot]

Q	A
Feature PR	symfony/symfony#47710
PR author(s)	@dunglas
Merged in	6.2

[wouterj]

We should document this new extensions option, which checks file extension and whether it matches allowed mime types, in the reference documentation for this constraint: https://symfony.com/doc/current/reference/constraints/File.html

@dunglas are you willing to open a PR for this? :)

[dunglas]

@wouterj done #17414