Choosing a mergetool according to file type

Dear all, 

Is there a way to specify which mergetool is called for different files in a repository in the same way that one can specify a diff tool using .gitattributes? I would like to use a custom mergetool for certain binary file types (MS Word dare I say it) but stick with my standard tool (FileMerge.app) for regular files.

I have set up custom mergetool and diff commands in .git/config:

[mergetool "wordmergetool"]
   path = /Users/jefferis/bin/
   cmd = wordgitmerge "$PWD/$REMOTE" "$PWD/$BASE" "$PWD/$LOCAL"
"$PWD/$MERGED"
[diff "wordgitdiff"]
   command = wordgitdiff

# adding the following to gitattributes handles the diff case *.doc diff=wordgitdiff

# but of course this doesn't alter the mergetool that is called. *.doc mergetool= wordmergetool

Many thanks for any suggestions,

Greg.

git version 1.6.3.2.202.g26c11 MacOS X 10.5.7