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.
Also, as this stands, it doesn't work anyway. The current code is checking if it is an instance of:
```
Illuminate\Validation\Symfony\Component\HttpFoundation\File\UploadedFile
```