Learn how to implement multiple file uploads along with HTML form elements in a portlet application by leveraging the features of the Java Portlet Specification V2.0 (JSR 286).

The JSR 286 Web Content Viewer portlet uses private and public render parameters in order to store information about which item to render. This information is stored in the URL until a threshold is exceeded and then only a placeholder is stored in the URL and the value is stored in the session. ...

Don't use relative URLs in portlet JSPs, if you want to reference a file in your portlet web app. They will be interpreted relative to the portal url by the browser and thus will not point to the resource you intend to embed in your markup.