See #10 If the format is csv I dont have to change the extension
If I set like default value xlwt then there is code that never is executed:

    try:
        import xlwt

        return 'xlwt'
    except ImportError:
        try:
            import pyExcelerator

            return 'pyexcelerator'
        except ImportError:
            try:
                import openpyxl

                return 'openpyxl'
            except ImportError:
                pass

See #10
Now works another time with python 3.3
Now works with django 1.3
Improvements in the test (fix test with python 3)

Fix #10