[Validator] Support "maxSize" given in KiB

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | symfony/symfony#11027
| Applies to    | 2.6
| Fixed tickets | symfony/symfony#10962


To display the constraint validation message with an expected suffix, this PR add a new option in  File constraint.
I also like suffix, since the user will use this as the suffix for their value. But the distinction actually hadn't occurred to me :)