Plupload
JavaScript API for building file uploaders
... as much of HTML5 as possible, we are able (among other things) to provide access to raw file data, even in such environments that do not normally support it. One of the biggest benefits of this is that we can display the thumbnails instantly, right as you select the images in the dialog or drag&drop them from the desktop. We slice the files in chunks and send them out one by one. You can then safely collect them on the server and combine into original file.