fixed validating of UploadedFile instance

I've added a new method isAValidUploadedFileInstance which checks for instanceof in both ways: with namespaces and without'em. Now using this function everywhere within class instead if direct checking for instanceof UploadedFile.
I've read the issue before commenting this PR
```php
use Symfony\Component\HttpFoundation\File\UploadedFile as SymfonyUploadedFile;
// then 
return $value instanceof UploadedFile || $value instanceof SymfonyUploadedFile;
```