viewfile no longer works with password protected files

When using viewfile (or now viewxls) to access an excel file that is password protected...

{viewfile:name=password protected access.xls}

... we see the following error:

viewfile: com.benryan.conversion.ConvertsionException: error converting spreadsheet

A copy of the same page/content works fine on our Confluence 2.10.3 production system. (No errors; password protected excel file contents are not rendered by viewfile macro, but the edit button will open this file as expected.)

Users were happy with how this worked on Confluence 2.10.3 production, but those pages break on Confluence 3.3.1 test system. So far, we have no viable workaround yet...