Hi @Kranthi Rohit 

Try bellow script in your action :

// Get the current logged-in user
var currentUser = gs.getUserID();

// GlideRecord to update the active comments field (assuming the table is 'incident')
var gr = new GlideRecord('incident');
if (gr.get(current.sys_id)) {
    // Update the comments with the logged-in user's ID
    gr.comments = "File attached by " + gs.getUserName();  // Add your custom message here
    gr.setValue('sys_updated_by', gs.getUserName()); // Update sys_updated_by to the logged-in user
    gr.update(); // Update the record
}