This section allows you to modify the extension of the filename. For most cases you will want to leave this checked on, with the option to 'Leave Unchanged'. If you do want to change your extension, you have several choices to work with. In the drop down list, you can see we can do:

Custom Extension: Allows you to manually specify a new extension. Once you select Custom Extension, you will be able to type in the field where it currently says .abc.

Upper Case: Automatically converts your extension to UPPER case

Lower Case: Automatically converts your extension to lower case

Proper Case: Automatically converts your extension to Proper Case, which means it will capitalize the first letter