From Comprehensible Input Wiki

To add icons that show if a link is mainly videos, audio, text, or an app, add one of these templates where you want the icon:

{{Pod}} for audio, {{Vid}} for video, {{Txt}} for text, {{App}} for apps, or {{Begin}} for content that starts at absolute beginner level. (If using Visual Editor, instead press Insert > Template in the toolbar, and search for the appropriate template.)

Or press the puzzle piece button in the toolbar to search for those templates.