How to strip extension from filename in dynamic stamp

I have a dynamic stamp that I want to populate the filename without the extension and add it to the meta data as well. I have tried several ways, but I just can’t seem to get it working. Here is the code I am using. The first line alone works fine for putting the filename in the field, but I can’t seem to display it without the extension.
 
event.value = event.source.source.documentFileName;
var foo = this.getField("ExhibitNumField").value;
foo = foo.substring(0, foo.lastIndexOf('.'));
event.source.source.info.foo = cMsg;
 
Any ideas?