Plupload is a JavaScript API for dealing with file uploads it supports features like multiple file selection, file type filtering, request chunking, client side image scaling and it uses different runtimes to achieve this such as HTML 5, Silverlight, Flash.
The Flash runtime uses a .swf file that can be built using the Flex SDK. This SDK can be downloaded from Adobe. http://www.adobe.com/products/flex/flexdownloads/
The Silverlight runtime uses a .xap file that can be built using the Silverlight SDK or Visual Studio. http://silverlight.net/getstarted/
You can read more about how to contribute to this project at http://www.plupload.com/contributing