There are some posts regarding the upload of local files to a JS API application. Most of them rely on a web-service for a) uploading the file and b) providing it to the JS API app via an URL. In general, security reasons and the HTTP protocol don't allow you to upload files to a JS application directly. A web-service - based on .NET, Java, PHP, Perl or anything else - allows uploading a file via HTTP. The Flex and Silverlight APIs behave different and the possibilities for accessing local data are greater. But for JavaScript I don't see a way for not using a service in the backend.
Hope that helps.
Axel