Plugin ?
The only thing i can do with plugins is importing them through admin control panel.
Anyway thanks a lot for your advice Lynne.
If any other friend have an idea about how to solve this problem, i will appreciate it a lot if he/she share it with me. Otherwise i will have to open 2 different categories with same content but with different attachments. So paid members will see the category with .txt attachments but all other members will see the category with .dlc extention attachments.
Thanks in advance...