I can't attach PDF files to a reply

- BotMan Version: v2.6.1
- PHP Version: 7.4.12

### Description:

this is the code:

```
hears('Send PDF', function ($bot) {

$attachment = new File('/pdf/example1.pdf', [
'custom_payload' => true,
]);
$message = OutgoingMessage::create('This is your PDF')
->withAttachment($attachment);
$bot->reply($message);

});
```
It shows the message "This is your PDF" but without attachment, I tried with the full url, but doesn't work either. The pdf folder is located in the public folder. I'm using Facebook Driver.

Start by reproducing the provided PDF attachment example with BotMan v2.6.1, PHP 7.4.12, and the Facebook Driver, then trace how OutgoingMessage and File attachments are handled by the driver. Done means the reply delivers both the text and the PDF attachment, including when the file uses a full URL.

Written by the indexing model from the issue text.