[Validator] Display a nice upload limit message

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | NA
| License       | MIT
| Doc PR        | NA

Actualy when a user upload a file who exceed the upload limit, the error messag looks like `The file is too large. Allowed maximum size is 2097152 bytes.` which is not very readable.
This PR use the same method who generate the `maxSize` error message by gessing the smallest possible number.
The error message became `The file is too large. Allowed maximum size is 2 MiB.`