Options System Snapshot / Excluded File Types page lists file types which must be ignored by the System Snapshot tool when taking snapshots.
A default list of Excluded File Types is available at the page. Make sure that there is a check mark next to those types you want to be ignored when taking or comparing a snapshot; remove the mark next to those which must be processed.
You can use the buttons under the list to add new Excluded File Types, modify existing ones or delete them. The wild characters * and ? can be used to define a set of similar types. Please note that you don't have to delete a file type just to stop considering it an Excluded one; instead, you can simply remove the check mark from the box next to it.
The button Default restores the default settings for this feature, including recovery of the items deleted by you and deletion of the ones added by you.