Adding extension for attachments

Another idea would be using [rel="enclosure"] (http://tools.ietf.org/html/rfc4287) from atom.

"The value "enclosure" signifies that the IRI in the value of the
href attribute identifies a related resource that is potentially
large in size and might require special handling.  For atom:link
elements with rel="enclosure", the length attribute SHOULD be
provided."