extension

This is the three-letter suffix part of a file's name:

filename.ext

The filename extension is important for telling Windows or CC3 what type of file it is. For instance, a CC3 drawing file ends in an FCW extension, while a bitmap file ends in a BMP extension.

Note that the extension simply indicates the file type. It does not determine the file type. You cannot convert a file from one type to another just by changing the extension. Changing a filename's extension is possible, but usually not appropriate.