Clean upload file names

[remik]

I also removed six.u() because it is working wrong on Python2 (there is a note: https://pythonhosted.org/six/index.html#six.u) eg. "АБВГДЕЖ.pdf" was converted to "Ð�Ð�Ð�Ð�Ð�Ð�Ð�.pdf".
Python 3.3 bring back support for u"" and Python 3.0-3.2 is no more supported.

[coveralls]

Coverage decreased (-0.06%) to 90.83% when pulling 5b1f8d5 on remik:feature/clean-filename into c91022b on batiste:master.

[batiste]

Great.
When you are at it could you add a few test cases:

filewithoutextension
filewith.two.extension
file<with></script>"'</script>malicious