Right now there is some kind of trouble with flash player. Today I have wasted more than two hours of my incredibly super important life because I couldn't upload an update for my mod. Later I have found info in support forum, that there is known problem with flash player. The browsers are blocking the flashplayer, and you can not upload a file because of that. To fix this issue you have to allow flash player. Like this, example from chrome: Select an option to allow addons for this time.