Having recently worked with this project I can say that it's pretty reliable and way better than most jQuery plugins you'll find out there.

The iframe transport method for uploads is useful for browsers that don't support XHR file uploads (basically IE < 10). You can read more a bit about how it works in: https://github.com/blueimp/jQuery-File-Upload/wiki/Frequentl... or directly in the source: https://github.com/blueimp/jQuery-File-Upload/blob/master/js... (it's pretty small).