/home/QtPalmtop/etc/mime.types is the file that you need to modify if you want to associate a different application with a particular file extension in the file browser. You must then reboot. I tested this by commenting out the:
application/x-troff-me me
line in the mime.types file and adding the me extension to:
text/plain asc me txt c cc h hh cpp hpp
Sometimes I see files named, "read.me", which should be opened as a plain text file. I found that I had to reboot to make this change active. Perhaps there is some way to do this without rebooting, but I do not know what that is.