Change Content-Type from application/octet-stream to multipart/form-data

multipart/form-data is usually used for html form submission, including file object if using html form.
API post request usually doesnt use html form format so wont be suitable for multipart/form-data.

I suggest you discuss with the app developer team, e.g. to use text/xml for xml post or application/json for json post, so more specific than general application/octet-stream.