tl;dr: Unchecked checkboxes usually don't send their value as
parameter when the form is submitted. The ajax request in our upload
code did just that which resulted in the value of checkboxes being
wrongly interpreted.

The default value of a checkbox is "on", even when it is off. The crux
is that with standard HTML forms, this value will only be included in
the form data if the checkbox is checked. The python end thus receives
"on" for a checked checkbox and the empty string for an unchecked
checkbox. It so happens that bool("on") == True and bool("") == False.

Our JavaScript code that sends the form data for the upload page
wrongly included the checkbox value, even if it was not checked. This
resulted in the python end always receiving "on".

See: http://stackoverflow.com/questions/33487360/formdata-and-checkboxes

fixes #106