Type guard in validators to narrow down the resulting type

[OoDeLally]

My form allows user to upload a file. At the initial state we have

type MyTFormData = {
  file?: File; // File can be undefined in the form
};
const form = useForm<MyTFormData>({
  defaultValues: {
    file: undefined,
  },
  onSubmit: async ({ value }) => onFormCompleted(value),
});

However I have a validator that ensures that file is set. The form is not submitted when file is undefined.
This is because the next steps of my app require a defined file.

type ValidFormData = {
  file: File;
}
const onFormCompleted = (value: ValidFormData) => {
  // Stuff
}

While my file field validator is indeed verifying that there is a defined file, the typing does not know it, and

onSubmit: async ({ value }) => onFormCompleted(value),

naturally yields an error, because MyTFormData is not assignable to ValidFormData.

Is there a way to make this guarantee carried across ? Maybe with typeguards in the validators?

Thanks