[xwiki-users] How to add attachments ?

Hi, how to add attachments using one of 2 methods: addAttachments(java.lang.String fieldName) and addAttachment(java.lang.String fileName, java.io.InputStream iStream) of com.xpn.xwiki.api.Document class ? I tried using them but don't know the right way to use. Please give me an example using these 2 methods.