James - I had a similar problem and, after some trials and searching found a bug report at: <
https://openacs.org/bugtracker/openacs/com/file-manager/bug?bug_number=28>.
To correct it, you can download updated code from the CVS server (guidelines are at <https://openacs.org/4/checkout> and put it in place by hand (command line or file manager).
From there, I updated the files in the Package Manager (and remounted in Site Map ... probably a bit of overkill). Anyway, it now works. Hope this will help you, too!
