Dynamic Stamp - text field

I am in the process of creating a dynamic stamp with a text field that has a custom calculation script to use the document name.

This is the script that I found and am using:

event.value = event.source.source.documentFileName;

It pulls the document name in fine but it displays it as 1234.pdf, is there a different script or more that I need to add to the current script that would pull in the document name but leave off the .pdf?