How to delete every file of a certain type ?

Hello, i am trying to free up more space in my external HD so i used Inventory X to see what was taking up space and where. Inventory X shows you how much spave is being taken up by file type aswell (ie, 20gb of AIFF audio files , 4GBs in Garageband Project files) . Inventory lets me delete files right form it, but only one at a time. I cant highlight a whole list using the mouse, using shift or command . Is there a way to delete every file of a specific type (ie,every PDF file or every mp3 file etc)? I can use spotlight to show me every file of a specific type, but i only want to delete files in my external HD (my internal drive has alot of free space) . Is there a way to do this either on Inventory X, or on the MAc itself ? thanks