Several validation rules with "OR" condition

I have the MyRequest class, which contain validation rules for request params. In the "file" field can be null or string or file. How I can build the rule conditions for it?

class MyRequest extends FormRequest
// ...
   public function rules()
        {
            return [
                 // 'file' => 'image|max:20480', // need to combine this
                 // 'file' => 'nullable|string', // and this through OR
                 // 'file' => 'nullable|string|image|max:20480' // <- don't working (string not pass validation)
            ];
         }
}

via Chebli Mohamed