The validation for an empty binary file (on the upload field) on the assets detail subapp should be similar as it is on M.-5-UI (old app).
To reproduce the issue, you must edit an existing item, removing its binary file with the "remove" button (the trash bin button).
Quoting avongunten (from slack)
the default behavior for field validation is
- field border turns red (1px)
- bubble below field shows message
In this case there is only a red star *.
But there also should be the bubble with the text "This field is required" ... or something similar.