[Validator] fix showing wrong max file size for upload errors

this was because the maxSize option wasn't parsed correctly and simple string comparision could lead to wrong results, e.g. 200 > 1000M

| Q             | A
| ------------- | ---
| Bug fix?      | [yes]
| New feature?  | [no]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | [yes|]
| Fixed tickets | [#6441,#5551]
| License       | MIT