You always need to perform a search first.
After the search results start coming in, click on one of the icons below the search bar to filter by the file type or file format you are looking for.
If you want to filter by file extension, click on the Search Tools button to the right of the screen, just above your search results and type your desired extension in the search window. You can use this feature to filter by name or even source. You can select/deselect the sources via clicking on the check boxes below the text bar or move the sliders below to adjust the number of seeds or file size.