code: func (v *Validator) RequireFile(field string) *ValidationResult {
  	if !v.data.FileExists(field) {
  		return v.addRequiredError(field)
  	}
  	bytes, err := v.data.GetFileBytes(field)
  	if err != nil {
  		return v.AddError(field, "Could not read file.")
  	}
  	if len(bytes) == 0 {
  		return v.addFileEmptyError(field)
  	}
  	return validationOk
  }
docs: // RequireFile will add an error to the Validator if data.Files[field]
  // does not exist or is an empty file