How can I change the file extensions Smart Selection is taking for backup?

You can achieve that in two ways:

 From your computer:

1.  Right click on the Revest icon in the system tray → Select Open Dashboard.
2. Click on Data Selection.
3. Refer to the Smart Selections tab.
4. You can change the file extensions that are backed up under: Office, Financial Files, E-Books and PDFs by clicking on the Edit button on any of these groups.
5. Add new extensions or delete what you don’t want Revest to backup.
6. You can edit any other data group by selecting it from the drop down menu in the top of the page.
7. Click on the tick to save the changes.

From Revest website:

1.  Sign in to your Revest online account.
2. Refer to the Policies tab.
3. Click on Global settings → Select the Smart Selections Extensions tab.
4. Select the category of the extension you want to edit → Add the new extensions separated with semicolons, or delete the unwanted one.
5. Click on Save to save the changes.