If i upload a .htaccess file to the root of my domain but then point the domains cname at tumblr will the contents of the .htaccess file be over ridden by tumblrs htacess file ?
Yes, it would.
The htaccess file is parsed and used by yout web server. By pointint your cname entry to tumblr, your web server is never involved at all. Hence tumblr's htaccess file(if any) will get invoked, not the one on your server.