specifying diff tool for rtf files

[Haszlakiewicz, Eric]

Is there a way to specify which diff tool to use for certain files?  I see that there's a merge-patterns section, but I haven't been able to get it to work.

[merge-patterns]
**.rtf = docdiff

"hg diff" from the command line just produces raw output, and using TortoiseHG's diff menu option starts kdiff3.

Perhaps there's something similar for diff?  I didn't see anything obvious in the hgrc page.

eric