Options Duplicate Files / Files To Find page defines the Duplicate File Finder's search scope.
You can select to look for duplicates among All Files, DLL Files or Files Named... (specific file types). To manage the list of specific file types use the buttons below it; make sure that check marks are placed in the boxes next to the file types you want to be processed.
Please note that identifying and deleting redundant files, especially among All or DLL files, is only recommended for experienced users! Always double-check your results before deleting anything. The golden rule here is if in doubt - don't delete!