Validator::validateMimes returns false if the uploaded file isn't valid

If the upload is partial, the UploadedFile instance might be invalid and the mimes validator shouldn't be valid for any mimetype.

Also the Symfony docs [1] states that only a few method on an `UploadedFile` have reliable results if the instance is invalid (`isValid()` is false), so i removed the call to `getPath`.



[1] http://api.symfony.com/2.4/Symfony/Component/HttpFoundation/File/File.html#method___construct