This adds the yaml file extension to the known textfile extensions for
the backend, so that i.e. form manifests are editable in the backend.

Resolves: #80654
Releases: master
Change-Id: I9306211c16ec83128c002e04ba30dd7d6c1f80be
Reviewed-on: https://review.typo3.org/52334


Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Cedric Ziel <cedric@cedric-ziel.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>