Firefox only emits Progress events until the file has been loaded, while Chromium and Internet Explorer emit Progress events up to and including 100%. The W3C FileAPI spec specifies that a progress event must be fired "at 100% completion of the read operation" ( http://www.w3.org/TR/FileAPI/#events )
Created attachment 8451359 [details] Testcase: Final reported loaded bytes Not sure if a testcase is required, but have this anyway to clarify: this outputs the Loaded Bytes count of the last Progress Event received before the Load Event. In Firefox, the result will be either 100% or less, depending on whether the whole file could be loaded at once. I haven't yet received less than 100% in other browsers (expected behavior).