SitePen reports that the Dojo Toolkit now has support for multi-file uploads, thanks to the new Deft project. The dojox.form.FileUploader class embeds a hidden SWF file in the page which, when triggered, will open a system dialog that supports multiple file selection, and also file masks, which allows the user to filter their selection by file type. If the user does not have version 9 or greater of the Flash Player installed it can, depending on the options you set, present the user with a standard HTML file input instead (or the option to install the latest Flash Player). The HTML form also supports multiple files, although due to browser restrictions, only one can be selected at a time. But they are all uploaded at once.
More info: Dojo multi-file uploader