[BUGFIX] fix swiftmailer namespace invocation

invocation of Swift_Attachment must be using
absolute path.

Resolves: #59894
Releases: 4.0
Change-Id: I0837194fad81a13d30fee80ff03976300dcca6cc
Reviewed-on: https://review.typo3.org/31278
Reviewed-by: Ivan Dharma Kartolo <ivan.kartolo@gmail.com>
Tested-by: Ivan Dharma Kartolo <ivan.kartolo@gmail.com>