Upload a file asset

Uploads a file and creates a file asset record. The request is a multipart/form-data body containing the file content and optional metadata (name, tags, format). Supported formats: CSV, PSV, JSON, JSONL, XLSX. Once uploaded, the file asset can be used as the data_source of a batch job via POST /v6/batches. The file enters AVAILABLE status on successful upload; use POST /v6/files/{fileId}/expire to release it when no longer needed.