Thank you for clarifying.
As I mentioned before, this was something that was
postponed, I just haven't had a chance to get around to it. I created a new
ticket for this since the old one seems to have vanished into thin air.
@leschek: Are you dropping the same file in IE11 and FF Portable? What type of file is it? Plupload currently ignores the dropped file if it doesn't have an allowed extension, without throwing an error (already reported to them, although they don't seem willing to change that behavior).