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Reserves an AI Application-scoped storage object and returns a short-lived direct-upload descriptor. Upload the file bytes straight to `uploadUrl` using the returned `method`, `headers`, and `fields`; the file never passes through Promptic. After uploading, POST to /storage-objects/{id}/complete to validate the actual stored size and MIME type. Declared size is not recorded as actual usage. Defaults to private access. MIME types are checked before signing: artifacts and benchmarks folders accept PDF, PNG, JPEG, WebP, plain text, CSV, Markdown, JSON, DOCX, and XLSX. Other folders additionally accept binary (application/octet-stream), PPTX, ZIP, GIF, SVG, and HTML. Media type matching is case-insensitive and ignores MIME parameters. Unsupported types return 400.