Thanks for the reply. In the screenshot above, one can see the raw multipart request from Soapui that has inserted a header for the attachment. I then downloaded the uploaded file from AWS S3 and a file "diff" with the original image shows the header has been included in the file content. So I gather AWS does not expect this multipart header. It counts everything after the message header as payload.
This did not work. Either I should be able to disable multipart attachments, or if the solution is to include the file as binary payload, prevent HTML encoding of the payload.
