Browse your space and click the Attachements Tab. Fill in the "Filter by File Extension" box and click Go. The filtering works (although it's case-sensitive), but the pagination is messed up. See the screenshots for examples. The results for page 2 and page 4 should be on the second page of results. It looks as though the attachment list is just hiding entries that don't have the extension you've chosen. It should be recalculating the results and shortening the list.