post https://api.notion.com/v1/file_uploads//send
Use this API to transmit file contents to Notion for a file upload.
For this endpoint, use a Content-Type header of multipart/form-data, and provide your file contents under the file key.
When mode=multi_part, each part must include a form field part_number to indicate which part is being sent. Parts may be sent concurrently up to standard Notion API rate limits, and may be sent out of order as long as all parts (1, ..., part_number) are successfully sent before calling the complete file upload API.
Include a boundary with the Content-Type of your request as per RFC 2388. Most request libraries (e.g. fetch, ky) automatically handle this as long as you provide a form data object but don't overwrite the Content-Type explicitly.