Files and photos to attach. Two modes: (1) External JSON metadata: set content_type='application/json; schema=el8_external_content_report' and provide 'external_content' JSON that includes a 'referenced_url' (and optional metadata). The JSON is stored and used to reference the external file; file bytes are not uploaded. (2) Binary upload: set content_type='application/octet-stream' and provide 'base64_content' (and optional 'original_filename'). When any file uses base64, all files in the same request must use base64. On GET, returns file references; download via the files retrieval endpoint. For JSON content, the retrieval endpoint returns the stored JSON metadata.
