[7.x] File::guessedExtension() method

[hivokas]

This PR provides a File::guessedExtension() method which could be useful in case we want to know the extension of the file which actually doesn't have one.

>>> File::name(public_path('image.png'))
=> "image"
>>> File::extension(public_path('image.png'))
=> "png"
>>> File::guessedExtension(public_path('image.png'))
=> "png"
>>> File::copy(public_path('image.png'), public_path('image'))
=> true
>>> File::name(public_path('image'))
=> "image"
>>> File::extension(public_path('image'))
=> ""
>>> File::guessedExtension(public_path('image'))
=> "png"

Under the hood, it guesses the extension by the mime-type.

It this PR gets approved, I'll provide tests.

[driesvints]

>>> File::guessedExtension(public_path('image.png'))
=> "png"

But your last example actually has an extension? I think that defeats the purpose?

[hivokas]

Sorry, that was a typo.
I updated the description.

[bonzai]

This will not work if I use Filesystem component outside of Laravel, because symfony/mime package is missing in composer.json:

https://github.com/laravel/framework/blob/7.x/src/Illuminate/Filesystem/composer.json

[GrahamCampbell]

Don't we already have a method for guessing extensions in the framework?

[GrahamCampbell]

https://github.com/laravel/framework/blob/v7.13.0/src/Illuminate/Http/Testing/MimeType.php