[HttpFoundation][File][UploadedFile] Fix guessClientExtension() method

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes

1) Change getMimeType() by getClientMimeType() (we are guessing extension based on client information after all)
2) Add use of missing namespace (ExtensionGuesser)
3) Add tests for getClientExtension() method