We have discussed earlier in one of the post was on Attachment file size limitations in CRM 2011
In this post I am discussing about ‘Manage Attachment file types in CRM 2011’
Users can attach any file to any record in Dynamics CRM 2011, except that file type is blocked in CRM 2011, when you try to attach a file, if that file type is blocked in CRM 2011, it gives an exception message “The attachment is not a valid file type”.
In the below example I am trying to attach an app.config file which is actually blocked in CRM 2011.
If you are the system administrator, navigate to Settings –> Administration and click on System Settings. You can set the file types to block in the General tab under ‘Set blocked file extensions for attachments’ shown below.
Microsoft Dynamics CRM Blog 
Is there also a way where the application can block an attachment when the filename has parentheses?
I don’t think so, as far as I know it doesn’t matter if filename has parentheses or not. I tried attaching the file which has parentheses and I could add in CRM 2011.