The extension is the part of the file name that comes after the basename. Under Windows, extensions are often (but don’t have to be) three characters long.

LinkFixer Advanced considers the period to be part of the extension. 

In some cases, a file name might contain more than one period, as in “my.file.xls”. For example, LinkFixer Advanced can be configured to recognize “.xls” or “.file.xls” as the extension. The options for defining what LinkFixer Advanced should use as the extension can be found in the "hamburger" menu | Settings | Files… in the “File Extensions” section.

Following is an example of a rename rule for the extension component:

Notice: Only include the dot (".") and extension itself in the "From:" and "To:" fields (without including the basename or other filespec components).