How to diff docx files with git difftool

Whereas it is not so difficult, to get docx files diffed through

git diff xxx.docx

i did not find a way to do so using git difftool which would be quite convenient. To recall configuration for diff:

• in gitattributes place a line *.docx diff=word, where word is just a placeholder
• in gitconfig place

[diff "word"]
  textconv = astextplain

Here, asplaintext is some converter converting .docx into plain text.

If i call git difftool xxx.docx these tools seem not to be applied. I would expect that the difftool is invoked with output of asplaintext, but it is not.

Am I too stupid to find a way, or is this just a gap in git?? That feature would be very nice because i use gitahead which invokes, as many other gui backends git difftool.