[FEATURE] Introduce $GLOBALS['TYPO3_CONF_VARS']['SYS']['mediafile_ext']

Releases: master
Resolves: #69543
Change-Id: I676a20fd64a13e6f04774fe2fa838c5b51d6eaf0
Reviewed-on: http://review.typo3.org/43045


Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Daniel Goerz <ervaude@gmail.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Tested-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Reviewed-by: Philipp Gampe <philipp.gampe@typo3.org>
Tested-by: Philipp Gampe <philipp.gampe@typo3.org>