Make the required fields instant visible instead afterwards while submitting some form or file. So that users know which fields are required.
Usually this is done with styling, for example by adding an asterisk (often with a contrasting color) to a label, or adding '(optional)' to a label when it is not required.
There's a nice article on Medium about how to approach this from an accessibility standpoint: Enhancing Accessibility in Mendix: Why WCAG Matters | by Sophie Bakker | Medium
Up once again ;)
Up up! :)