DZI filename generation removes extension twice

[Tevemadar]

A set of files having increasing amount of dots inside can show this.

red_v1.png
green_v1.2.png
blue_v1.2.3.png
purple_v1.2.3.4.png

will produce correct folders (.png extension removed, _files appended):

red_v1_files
green_v1.2_files
blue_v1.2.3_files
purple_v1.2.3.4_files

but incorrect descriptors:

red_v1.dzi
green_v1.dzi
blue_v1.2.dzi
purple_v1.2.3.dzi