Send Mail with attachments

Hello,

I have a small application where I create invoices. One feature of the app is to send the invoice as pdf attachment via mail to clients. This worked great. Now I added the option to add further pdf files to the mail.

I use a Mailable like this:

<?php

namespace App\Mail;

use App\Models\Client;
use App\Models\Invoice;
use Illuminate\Bus\Queueable;
use Illuminate\Mail\Mailable;
use Illuminate\Queue\SerializesModels;
use Storage;

class NewInvoice extends Mailable
{
    use Queueable, SerializesModels;

    /**
     * Create a new message instance.
     *
     * @return void
     */
    public function __construct(
        public Client $client,
        public Invoice $invoice,
        public string $emailText,
        public string $additionalAttachmentsPath
    )
    {
        //
    }

    /**
     * Build the message.
     *
     * @return $this
     */
    public function build()
    {
        $user = $this->client->user;
        $mailable = $this->from($user->email, $user->name)
            ->view('emails.new-invoice')
            ->with([
                'user' => $user,
                'emailText' => $this->emailText,
            ]);

        if ($this->additionalAttachmentsPath !== '') {
            $additionalAttachments = Storage::files($this->additionalAttachmentsPath);

            foreach ($additionalAttachments as $additionalAttachment) {
                $mailable->attachFromStorage($additionalAttachment);
            }

            Storage::deleteDirectory($this->additionalAttachmentsPath);
        }

        $mailable->subject(__('New invoice'))
            ->attachData(
                $this->invoice->getPdf()->output(),
                $this->invoice->fileName,
                [
                    'mime' => 'application/pdf',
                ]
            );

        return $mailable;
    }
}

This is then used in a service as follows:

class InvoiceService
{
    public function sendInvoice(
        Invoice $invoice,
        Client $client,
        string $emailText = '',
        array $emailCC = [],
        string $additionalAttachmentsPath = ''
    )
    {
        $mailable = new NewInvoice($client, $invoice, $emailText, $additionalAttachmentsPath);

        try {
            Mail::to($client->email)
                ->cc($emailCC)
                ->bcc($client->user->email)
                ->locale($client->language)
                ->queue($mailable);

            $invoice->update([
                'sent_on' => today(),
            ]);
        } catch (TransportExceptionInterface $exception) {
            Log::error($exception->getMessage());
        }
    }

	...
}

Now the problem is that I always get the following error, when I add additional attachments:

The body must be a string or a resource (got "null").

As you can see I use PHP 8.1 and Laravel 9.20 for this application.

I already spend a whole evening to search for what could be the reason, but couldn't find anything. Hopefully I get some help here. It's very much appreciated.

Thanks, Michael