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I just tried to upload a 5kB GIF image to the question I'm composing but it is still saying 'Uploading ...' even after more than one minute!
It was due to Adblock Plus Firefox plugin! When I disabled it for this particular site, the problem dissapeared!
Note that I hadn't this problem with Stack Overflow site, which is quite confusing so I didn't come to the idea it could be caused by Adblock Plus.