A required element must have at least one file uploaded by the user before the form can be submitted.
Making an element required
File Upload → Settings
Tick the box at the Required field to make it required.
Required error message
File Upload → Settings → Optional
Change the required error message by entering text into the fields under Translate error messages.
Required indicator text
Settings → Style → Labels
Change the text indicating the element is required. By default this is (required), but you may want to make it an asterisk * for example.