[5.2] Added a "file" validation rule

[themsaid]

As requested here #13301

The new rule checks if the given value is a valid file.

[taylorotwell]

Updated to just:

    protected function validateFile($attribute, $value)
    {
        return $this->isAValidFileInstance($value);
    }

[themsaid]

I should have done it that way, I wonder why I did not! :)