Normally PD4ML extracts a file name from attachments URL. In your case an attachment body is passed as a base64-encoded stream of bytes with no file name hint – in such situation a description attribute with a file name is needed.
In the forthcoming v398fx3 we implemented an “auto-guess” of file extension by a mime type. If there is no description, it will extract a mime type from “data:” prefix.
auto-names the attachment as “attachment.txt”