Occasionally Bulk Import API calls will "hang" or "stall" never returning a response. This may appear to be a blank or missing response when the API is actually still waiting for the end of the file to be transferred.
You can add the following header:
to your API calls and it will fix this issue. Please note that adding this header on the client side will cause cURL to send data in chunks. For more information on chunked file transfer, see Wikipedia: Chunked Transfer Encoding