[6.x] Fix validating image/jpeg images after Symfony/Mime update

[Grldk]

Symfony/Mime v5.20 changed the returned extension for an image with image/jpeg mime type: symfony/mime@aa1d922

This breaks laravel image validation, as it relied on a return value of .jpeg. This commit adds .jpg, which fixes the validation.

(see #35416)

[GrahamCampbell]

Note to reviewers: Laravel 6 requires Symfony 4 components, but those in turn allow both Symfony 4 and 5 for their own dependencies. The mime dependency is one of those, which is why this really does need patching on 6.x.

[patrickomeara]

Just a follow up note to this.

Previously mimes:jpeg would accept jpg and jpeg, but now won't accept either. mimes:jpg must be used to accept both.

The breaking change is in https://github.com/symfony/symfony/pull/38407/files#diff-d9bc28406dbb260810697db93dd597ace181aa0a6b60fc920c3ab5da92203c1eL1323

Swapping the values back to what they were before the change ie. 'image/jpeg' => ['jpeg', 'jpg', 'jpe'], brings back the previous functionality.

I believe Laravel should accept both jpg and jpeg if either are mentioned in the mimes:

At the moment the example snippet in the docs won't accept either jpeg or jpg. https://laravel.com/docs/master/validation#basic-usage-of-mime-rule

[driesvints]

Thanks @patrickomeara. I've sent in a PR to the docs to fix that as well as a PR to reintroduce the previous behavior for the mimes rule: #35518