There is actually a very simple way, Here is the way you can do it:
var xml = '<xml>'+
'<number>'+current.number+'</number>'+
'<state>'+current.state+'</state>'+
'</xml>';
var attachment = new Attachment();
var attachmentRec = attachment.write('incident', current.sys_id, "record.xml", "text/xml", xml);
gs.addInfoMessage('An attachment is created with sys_id and attached to this incident'+ attachmentRec);
This is the idea.
