[BUGFIX] Allow Video Text Tracks files

This patch allows the selection of vtt files in EXT:mediace as caption file.
Additionally, the mandatory "default" attribute is rendered.

Resolves: #65936
Releases: master, 6.2
Change-Id: I7f94857150503e6c03713a61fa3a4d2e093cf326
Reviewed-on: http://review.typo3.org/38102


Reviewed-by: Carla Froitzheim <cf@computer-foto.de>
Tested-by: Carla Froitzheim <cf@computer-foto.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Andreas Fernandez <andreas.fernandez@aspedia.de>
Tested-by: Andreas Fernandez <andreas.fernandez@aspedia.de>