Writing test for function to store files

I have a function in a trait which stores an attachment to a Google Cloud Storage bucket like:

  public function storeFiles($type, $id, $attachments)
  {
    $folder = $type . '/' . $id;
    $disk = \Storage::disk('gcs');

    if (!empty($attachments)) {
      $output = [];
      foreach ($attachments as $attachment) {
        $file = $attachment['file'];
        $filename = $file->getClientOriginalName();
        $file->storeAs($folder, $filename);
        array_push($output, [
          'name' => $filename,
          'notes' => $attachment['notes']
        ]);
      }

      return $output;
    }
  }

This can be called for example like:

$this->storeFiles('avatars', $user->id, $request->all()['attachments'])

I want to test this function, and I am reviewing the docs from https://laravel.com/docs/8.x/http-tests#testing-file-uploads

Q1. Should I write a unit test or a feature test?

• I am guessing Feature test

Q2. Do I have to edit the phpUnit.xml file to use local storage or is there a way to simulate the cloud storage bucket?

• I am guessing I can use Storage::fake('avatars') to produce a "fake" local storage, but I worry that since I specify the disk in the storeFiles function, it may supersede it?