Despite the words of caution, but eager to get my hands on the workbooks, I tried using FireFox to download the workbooks and it works well (but not like you'd expect). Just click the link of the challenge, hit the Challenge-button at the bottom of the workbook. Now comes the trick... click with your right mouse button in the frame with the gobbledygook, select 'This frame »' and download using 'Save Frame As'. Workbook is now downloaded as .twbx.