[TASK] File: add getNameWithoutExtension() method

There is currently no way to get the filename without its extension,
e.g. to create a new name for a file converted to another format. This
commits adds such a method, together with tests and a fix for names like
somefile.tar.gz.

Resolves: #41768
Releases: 6.0
Change-Id: Iad924d1be4100cf798e36f25a22a32b221b8aef7
Reviewed-on: http://review.typo3.org/15427
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter