Hi There,
How are you firing your POST? Using curl? Postman?
It is necessary to have a folder pre-existing where you want to upload an image.
For example I use following command from curl console and it works:
curl http://localhost:4502/api/assets/my-image123.jpeg -H"Content-Type: image/jpeg" --upload-file "preview_Monitor_template.psd" -v --user admin
Try --upload-file to pass binary file.
Thanks!