Problem: Unable to upload in IE9 from outside my internal network. Looks like it has to do with Flash add on.

I am uploading a 290MB file. It freezes when progress bar is between 70% and 95% complete
I am using IE9 on Win2008R2 computer. IE has minimal addons - Flash, AdobeReader and MSXML.
Workarounds for this behavior
Workaround 1. If I revert to compatibility mode it works fine
Workaround 2. If I disable Flash addon it works fine

Note: it also works fine with other browsers (FF and Chrome)

Conclusion: problem in IE 9 with Flash add on.  (IE9 uses latest Flash version 11.x)