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Most portals can't handle portlet content types other than 'text/html'. The reason is that the content has to be rendered inside the overall web page of the portal (which is generally in html) and it has no way to render other mime types within that page. So, in order to download a dynamically generated file (such as a Excel spreadsheet or a PDF) or to include a dynamically generated image in your portlet, you have to create a link to a non-portlet URL that will return the desired content. This is usually done with a traditional servlet from within the same webapp as your portlet. There are some important factors to consider when doing this:

• Security: If the content coming from your servlets is at all sensitive in nature, you need to make sure they are secure and that a cut-and-paste of a URL doesn't give direct access to unauthorized users. If you are currently relying only on portal-based security, then this can be a problem. You will need to make sure that the servlet can properly verify the authentication and authorization of the user. The most likely way to do this is by sharing some session data between the portlet and servlet.

• Sessions: If you need to share session data between the portlet and the servlet (which is likely when addressing the Security issue) then you need to make sure your portlet and servlet containers are capable of this. Take a look at How Do I Share Session Data Between Portlets and Servlets in my Webapp? for more information on this.