Getting attachment from mailgun

Unsure if this is the right place to ask but I'm trying to setup inbound emails. I've got it so my application accepts emails via a webhook and now I'm trying to use the Guzzle client to request the file and store it into my local storage folder.

    {
        
        $files = collect(json_decode($request->input('attachments'), true))
            ->filter(function ($file) {
                return $file['content-type'] == 'application/pdf';
            });

        if ($files->count() === 0) {
            return response()->json([
                'status' => 'error',
                'message' => 'Missing expected PDF attachment'
            ], 406);
        }

        foreach($files as $file){
            $response = (new Client())->get($file['url'], [
                'auth' => ['api', config('services.mailgun.secret')],
            ]);
           $pdf = $response->getBody();

            Storage::put($file['name'], $pdf);
        }

        return response()->json(['status' => 'ok'], 200);
    }

This is the code that is not working:

$response = (new Client())->get($file['url'], [
                'auth' => ['api', config('services.mailgun.secret')],
            ]);

Example of what is stored in $file['url']

https://se.api.mailgun.net/v3/domains/sandboxf0ce04dxxxxxxxxxxxxxxx.mailgun.org/messages/eyJwIjpmYWxzZSwiayI6ImZxxxxxxxxxxxxNDYxZi1iNTQ1LWUwOWM2NTQ3NjA0MyIsInMiOiIxYWFiMWI1MTQ3IiwiYyI6InRhbmtiIn0=/attachments/0

Error:

[2019-01-05 03:36:50] local.ERROR: Client error: `GET https://se.api.mailgun.net/v3/domains/sandboxf0ce04d10b4b4c1287962dda52b3e7a6.mailgun.org/messages/eyJwIjpmYWxzZSwiayI6ImZhNjI5NTY1LTMwYjUtNDYxZi1iNTQ1LWUwOWM2NTQ3NjA0MyIsInMiOiIxYWFiMWI1MTQ3IiwiYyI6InRhbmtiIn0=/attachments/0` resulted in a `401 UNAUTHORIZED` response: Forbidden

Now I understand that for some reason it's not authenticating correctly but unsure why when I'm passing through the correct Application Secret Key. I've read on the internet it may just be a sandbox issue but wanted some insight before attempting live.