I am having the same issue. It doesn't upload the files for Safari 7.0.4 or 7.0.6, nor for Opera 24 nor Firefox 30, 31 or 32.
On Safari it does show an IO, Server error briefly right where the progress bar would appear. But for the other browsers I mentioned, it shows uploading progressing via the progress bar, but when it's done, no files are attached to the form. I'm not sure where to look for the solution.
It works fine with Chrome 37 and IE 11.