All file paths use forward slashes to separate path segments, independent of operating system. Resin will translate the path to the local filesystem path. Just separate them by commas. It also lets script set mail headers by setting query variables. Certain mail headers are protected, e.g. Sender and Date .
Scripts use a normal write stream to send the message body.
Just open the URL using openWrite and use the normal stream functions. When the stream is closed, Resin will send the message. Thanks for creating an awesome product. Getting a file is a single HTTP operation. Optimizations such as keepalives and caching are handled transparently by Resin.