In addition, in order to include an attachment with this kind of web service, you need to adjust how you package the information (ie: not a URL encoded parameter list, but instead a multipart message with proper attachment encoding) as well as changing how you call the webservice (content encoded, rather than URL encoded)
The web service function expects not only a content section, but also a content type. The example shows application/xml and application/json.
For this use case, we have to specify that it is a multipart message (including the boundary)
here is an example, sample content section which includes the email, recipients, message body, and a CSV attachment.