$array = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20];

$t = "";
foreach ($array as $i)
{
    $t .= $i . PHP_EOL;
}

IlluminateSupportFacadesStorage::disk('public')->put('400.txt', $t);

'public' => [
        'driver' => 'local',
        'root' => storage_path('app/public'),
        'url' => env('APP_URL').'/storage',
        'visibility' => 'public',
    ],

<?php namespace AppMail;

use IlluminateBusQueueable;
use IlluminateMailMailable;
use IlluminateQueueSerializesModels;
use IlluminateContractsQueueShouldQueue;

class MailTxt extends Mailable
{
    use Queueable, SerializesModels;

    public function __construct()
    {
        //
    }
    public function build()
    {
        return $this->view('email_txt')
                ->from('email@com.pt')
                ->attach(public_path('/storage/400.txt'));
    }
}

$array = [
  1,2,3,4,5,6,7,8,9,10,
  11,12,13,14,15,16,17,18,19,20
];

$t = "";
foreach ($array as $i)
{
    $t .= $i . PHP_EOL;
}

IlluminateSupportFacadesStorage::disk('public')->put('400.txt', $t);

IlluminateSupportFacadesMail::to("parapua@sapo.pt")->send(new AppMailMailTxt());