Two different rules on Post method with FormRequest in Laravel

Question

I use Dingo API and FormRequest to validate HTTP API requests. I have those rules:

    switch ($this->method()) {
        case 'POST':
            return [
                'file' => 'required|mimes:pdf,png,jpeg,jpg',
                'title' => 'required|unique:documents'        
            ];
        case 'PUT':
        case 'PATCH':

            return [
                'id' => 'required',
                'filename' => 'unique:documents,filename' . $this->route("id"),
                'title' => 'unique:documents,title' . $this->route("id")
            ];
        case 'DELETE':
            return [
                'id' => 'required'
            ];
    }

However, I realised that I need to have two different rules in the case of POST. If a certain variable exists on the request, then I don't need the file to be required.

For example if collected is true or exists on the object I send with HTTP request, then file should be required. Otherwise not.

My fix at the moment is to split those two in different controllers, but I am not satisfied and I was looking for a tweak on the actual rules.

Any idea?