There should be an implementation for your server.
For J2EE servers, I'd recommend Jakarta Commons FileUpload though many frameworks, like struts, ship with it.
PHP has an implementation info is in the manual. Just beware, there have been many security vulnerabilities in the PHP implementation.
If you are using Microsoft IIS you can find some info as well.
Roxen just stores the uploaded file as a request parameter.
the list goes on and on.
So, the answer really just depends on what web/application server you might be using.
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Paul Davis willCode4Beer.com (coding for all the right reasons)