Media payload to probe. Mounted through WORKERFS, so no main-thread byte copy is made.
OptionalneedsSet for formats the probe converts in place (HEIC). WORKERFS is read-only, so the payload is copied into the worker's own writable FS instead of being mounted — matching what the main worker does for these formats. Costs one byte copy, so only set it when needed.
Parameters describing one pooled media info probe request.