Instead of converting the uploaded file object to an UploadedFile
instance from Symfony, because the latter is compatible with
Laravel's validation, let's re-implement the validation for the
three rules we were using.
The benefit: we can now avoid copying the uploaded file to a
temporary location just to do the wrapping.
In the next step, we will remove the temporary file and let the
uploader / Intervention Image handle the PSR stream directly.