Mailgun API - Send URL as attachment in Send Email not working

Question

I am trying to send url as attachment using Send Email api in Mailgun. But the attachment is not attached. I am using Laravel HTTP client

$params = [
            'from' => $selected_options['mg_from_email'],
            'to' => $selected_options['mg_to_email'],
            'subject' => $selected_options['mg_subject'],
            'text' => $selected_options['mg_text'],
            'html' => $selected_options['email_body'],
        ];

$response = Http::asForm()->withOptions(['verify' => config('constants.verify_ssl')])->timeout(30)->withHeaders(
            [
                'Content-Type'  => 'multipart/form-data',
                'Authorization' => 'Basic ' . base64_encode('api:'.$api_key)
            ]
        )->attach(
            'attachment', file_get_contents('https://picsum.photos/200'), 'photo.jpg'
        )->post(
            $request_url,
            $params,
        );

Can anyone suggest what i am doing wrong here?

There is no error in response and the email is also sent successfully but the attachment is not there.