A extensions validation rule for Laravel 5

validation.php

<?php

// For example in AppServiceProvider
Validator::extend('extensions', function ($attribute, $value, $parameters) {
    return in_array($value->getClientOriginalExtension(), $parameters);
});

Validator::replacer('extensions', function($message, $attribute, $rule, $parameters) {
    return str_replace([':attribute', ':values'], [$attribute, implode(',', $parameters)], $message);
});

// Usage

Rule: extensions:extension1,extension2
In context: 'file' => ['required', 'extensions:extension1,extension2'],