Suggestion: Use the __ia_thumb.jpg file as a default thumbnail

[markpbaggett]

While reviewing audio collections, I noticed that most audio manifests lack thumbnails. This is likely because thumbnail generation that creates files that match format="Thumbnail" relies on a PDF, video, or image.

It would be helpful if audio files consistently had thumbnails, similar to how they appear on Internet Archive pages—whether derived from a user-uploaded image or generated from a spectrogram.

To ensure this, I propose we also look for file['name'] == "__ia_thumb.jpg".