Advanced Filters version 1.5.0.0 now has an integrated Furnishing Materials subfilter!
This is a plugin using the Advanced Filters extensible dropdown filter system. It adds a furnishing material filter that can be accessed when Materials >> All is selected.
NOTE: This plugin was intended only as a stopgap until integrated functionality (a dedicated subfilter button) was added to Advanced Filters. That integrated functionality was added with AF version 1.5.0.0. This plugin's functionality is now redundant. I have no plans to update it.