Hi Mark, thanks for getting back to me, please find body params below for a 1KB jpg image which produces the error. Please let me know if you require any further information.
{"attachment":{"mime_type":"image/jpg","id":null,"file_name":"test.jpg","file_extension":null,"file":"/9j/4AAQSkZJRgABAQEAYAB..."description":null,"attachment_context_type_id":"PURCHASE_INVOICE","attachment_context_id":"f62882f36e7e4ea09dce2275f2fa3b4e"}}