Attachment in web applicaiton

hi can any one tell me how code for multiple attachment .What i mean is if user wishes to add another attachment he will be clicking on a link or button next to the first Attachment , if effect of this another attachment box appears to attach(Similar to our gmail ). how can this be done.

This code what i have written is for single attachment.

What i want is some link or button next to the first attachment , on clicking the link it sound launch another attachment box to attach.

Attachment FileUpload