Laravel Mail Not Sending Attachments Correctly After Update from 5.5 to 5.6

I thought I had checked over everything when it came to preparing for an upgrade but now I've run across a strange issue. In the past, this script worked well for me:

                foreach($remitContacts as $rc){
                    $mail = Mail::send('emails.manifests.agentManifests', ['carrier_manifest' => $carrier_manifest], function($message) use ($carrier_manifest,$rc,$storagePath){
                    $message->from('****', '*****');
                    $message->subject('Agent Manifest - '. $carrier_manifest->manifestNumber);
                    $message->to($rc)->cc('******');
                    $message->attach(public_path() . '/' . $storagePath, [
                        'as' => 'manifest.pdf',
                        'mime' => 'application/pdf',
                        ]);
                    });
                }

But now when it sends, the email is correct, but the attachment it sends is nothing more than text added to the body, and looks like below:

Does anyone have any suggestions? I wasn't aware this would be affected but apparently was...

Thanks!

via Chebli Mohamed