[TASK] Suggest "fileinfo" PHP extension

While not exactly mandatory this extension is essential for a fully
working TYPO3 installation, thus recommend to install it.

Resolves: #77153
Releases: master, 7.6
Change-Id: Ifa88653e36a86771b7549f953a019d2cee0bd367
Reviewed-on: https://review.typo3.org/49250


Reviewed-by: Frederic Gaus <frederic.gaus@flagbit.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>