How to change default permission for uploaded files in apache with mounted webroot?

I have an ubuntu server 11.10 with apache 2.2.20, php 5.3.6 and an installation of Joomla cms. I have used an extra hard disk as my web server storage and mounted it into /data/www/ (I hope it's not where my problem us!).

I've set permission of all files and folders in my web root to 755 and user groups for them is set to [default ubuntu user(in my case radio)]:www-data.

In past days I had serious problems with joomla not showing new uploaded images and other files and also I can't install any extensions. After hours of searching I found out that uploaded files don't have appropriate permission (they are -rw-------) and Joomla application cannot read, copy or move them after upload.

I’m wondering how can I set a default permission so all files that I upload use it?

PS: I’ve tested umask but it did nothing. I think it has nothing to do with my problem.