With a custom upload control, you would just attach the validation to the real (hidden) file input then reflect any error statuses beside the "fake" input the same as illustrated here.
Im not sure about checkbox groups, but Ill do a little research and experimentation and see what I can discover.
After digging in their github issues / spectrum chat for a little while, I was able to find an example that I think is what you are looking for in regards to checkboxes.
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