Wire placeholders (handler view)
Before executeRPC calls your handler, transports run materialization: JSON-safe placeholders in args are replaced with real File, Blob, or Buffer values according to catalog metadata and host options.
Accepted placeholder shapes
Same rules as Protocol file uploads:
__qaFileRef/__qaFileRefs/ array-of-ref-objects for ref mode.__qaFileUrl/__qaFileUrls/ array-of-url-objects for URL mode.
Placeholders must appear only at catalog-approved argument indices or nested paths (filePaths / fileArrayPaths).
Failure codes
When materialization fails (unknown upload id, fetch error, mixed ref+url leaf), transports respond with UPLOAD_MATERIALIZE_FAILED — see RPC error codes.