Look up a cached thumbnail by file id. Returns undefined when missing or on error. Thumbnails are keyed by fileId alone — writes (e.g. a whiteboard saving a fresh snapshot) naturally overwrite the prior entry.
Look up a cached thumbnail by file id. Returns undefined when missing or on error. Thumbnails are keyed by fileId alone — writes (e.g. a whiteboard saving a fresh snapshot) naturally overwrite the prior entry.