Solution: it seems the ifilters (dlls used to open files of certain types) may become unregistered as you install other programs that can be associated with those file types. This worked for me for rtf:
regsvr32 C:\Windows\System32\rtffilt.dllSee: RTF file searching is not working? - Mythicsoft QA

I assume that knowing the other ifilters (which can be listed using Nirsoft's SearchFilterView) one can re-register them.

It follows that the tpl file type may not have an appropriate ifilter registered for it- or that filter may not be registered.