@sinu2 is this ui action on which table ? i assumed it was on demand ,so suggested this logic

sendAtt: function() {
var source= this.getParameter('sysparm_defect_sys_id');
  var gr= new GlideRecord('sys_attachment');
  gr.addQuery('table_sys_id', source);
   gr.addQuery('table_name', 'rm_demand'); 
  gr.query();
   while (gr.next()) {
 var attachment = new GlideSysAttachment();
var copiedAttachments = attachment.copy('rm_demand', source, 'rm_story', this.getParameter('sysparm_story_sys_id'));
gs.log('Copied attachments: ' + copiedAttachments);
        }
},

///refer doc related to : GlideSysAttachment - copy(String sourceTable, String sourceID, String targetTable, String targetID)