webwork.multipart.saveDir is not honoured (anymore?)

Raised from BSP-3763.
This variable is used to point to a directory used to process some (depending on submitted content-type) HTML forms. It seems that it does not (always?) work.
If this variable does not work, standard (using supported OSes) installations of Bamboo may become broken until restart: some forms stop working and trigger internal server errors for no obvious reason.