Added .pdf extension to the thumbs/previews supported formats (#11514)
Added .pdf extension to the thumbs/previews supported formats (#11514)

- The extension was accidentally removed in a previous commit.

Acidburn0zzz/sdk,Acidburn0zzz/sdk,meganz/sdk,meganz/sdk,Acidburn0zzz/sdk,Acidburn0zzz/sdk,meganz/sdk,Acidburn0zzz/sdk,Acidburn0zzz/sdk,meganz/sdk,meganz/sdk,meganz/sdk,Acidburn0zzz/sdk,Acidburn0zzz/sdk,meganz/sdk