It looks like the function name(s) to send messages with attachments may have changed but the documentation wasn’t updated.
Try:
actions.get("mail", "mail:smtp:xxxx").sendHtmlMailWithAttachment("myemail@example.com", "Subject", "Message", "file:///some/path/to/a/file.jpg");
or
actions.get("mail", "mail:smtp:xxxx").sendHtmlMailWithAttachments("myemail@example.com", "Subject", "Message", arrayListOfStrings);