Creating file attachment with REST API fails when following sample code

Content

When attempting to create an attachment with the new REST API, it fails:

I follow the doc (http://documentation.custhelp.com/euf/assets/devdocs/november2015/Connect_REST_API/wwhelp/wwhimpl/js/html/wwhelp.htm#href=Connect_REST_API.1.18.html), doing a POST to https://xxxyyyzzz.custhelp.com/services/rest/connect/v1.3/incidents/20/fileAttachments with body from the doc example:
{
"fileName":"SampleFileAttachment.txt ",
"data":"c2FtcGxlIGNvbnRlbnQgZm9yIGZpbGUgYXR0YWNobWVudA=="
}

The response is a 400 Error:
{
    "type": "https://xxxyyyzzz.custhelp.com/services/rest/connect/exceptions/OSC-CREST-00012",
    "title": "Problem creating resource",
    "status": 400,
    "detail": "Error attempting to create resource: File specified could not be found: /tmp/c2FtcGxlIGNvbnRlbnQgZm9yIGZpbGUgYXR0YWNobWVudA==",