Required Validator message shown on FileUpload although data is added

Description

Steps to reproduce

1. Create a multistep form:
_FormStep1 : FileUpload(required validator), Element1,
_ FormStep2: Element2..
2. Add the form to a page then publish.
3. View the page.
4. Enter data on FileUPload then click Next step > Data of FileUPload is saved.
5. Click the Previous step button.
6. Edit data on Element1 (or do nothing).
7. Click Next step.

Expected: No validator message.
Actual: Required message is shown on FileUPload.