FAL needs mime type info. Use fileInfo functions here and make
fileinfo PHP extension mandatory.

Change-Id: If6fc39d4b42e0264927ecf6918e7deeb7ef9823c
Fixes: #36706
Releases: 6.0
Reviewed-on: http://review.typo3.org/10914
Reviewed-by: Tolleiv Nietsch
Tested-by: Tolleiv Nietsch
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers