The issue is that while the value of a single selection field will be the option that was selected, the value of a multiple selection field is just a number with bits set to indicate the selections. So instead of using the value as part of the file name, you need to test for which bits are set and build the entire name yourself. Hope that makes some sense - here's an article that Lynne wrote on using profile fields in the postbit:
https://vborg.vbsupport.ru/showthread.php?t=250418