[BUGFIX] Missing signals on adding file

There are signals for most operations performed in the file storage, but
not for adding a file. Especially this operation is interesting for
things like file indexing, automatic generation of variants (renditions)
etc., therefore this patch adds the two missing signals.

Change-Id: I0def7b44ff8977a42916e01388604fcbf2d6d299
Resolves: #50795
Release: 6.2, 6.1, 6.0
Reviewed-on: https://review.typo3.org/22823
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind