I am sorry, made a mistake.
'Reverted' the change on _format_string
While using SanitizeFileName on the endresult solves the dot problem on filenames,
it also removes / and \ in the endresult. Creating another issue.
For now it's better to keep multi subdirs functionality and find another way to solve the dot issue.
SanitizeFileName also removes / and \ bad for creating subfolders