When you type in the layer's name, follow it with a period. Everything after this period becomes the extension.
wall.int
wall.ext
Wall.site.D
site.D is all in the extension
Erika
Architect, Consultant
MacBook Pro Retina, 15-inch Yosemite 2.8 GHz Intel Core i7 16 GB 1600 MHz DDR3
Mac OSX 10.11.1
AC5-18
Onuma System
"Implementing Successful Building Information Modeling"