[HttpFoundation] UploadedFile - moved some code from move() to isValid()

| Q             | A
| ------------- | ---
| Bug fix?      | [no]
| New feature?  | [no]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | [yes]
| License       | MIT

I think ```isValid()``` should check if it's a real uploaded file instead of having this only in ```move()```, because one could also do something else with the uploaded file than moving it (create a thumbnail from it for eg.).