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.
@lucasmichot 
I've tested it and the only way ot works is using FQCN. Looks like a PHP bug

@GrahamCampbell 
Yeah, my bad. But it worked surprisingly.