Finalise a chunked PPTX upload

Concludes a chunked upload. fileIdentifier must match the requestIdentifier used when uploading the chunks via POST /api/v1/template-converter/preprocess.

The finalised file must be a .pptx, no larger than 200 MB, and no more than 100 pages. Runs before the conversion's callback_id exists — the fileId it returns is what you pass to POST /api/v1/template-converter/start.