Alain,

I did not have good luck with modifying the plist files for TextMate or TeXShop either.  However I installed Ciaran's excellent Quicklook plugin, and modified the plist file for the plugin to include org.tug.tex.  Complete instructions are on his blog.  http://ciaranwal.sh/

Make sure that you restart Finder, or run  qlmanage -r   from the command line.  I think glmanage is the most reliable way to make sure things work.