First make sure the device is up to date – check www.t-mobile.com/bbupgrades that you are running the newest sw. then check the following:
Verify the attachment module is installed, from the home screen open Options and select Applications.
To install the BlackBerry Attachment Service using the Application Loader in the BlackBerry Desktop Manager.
I figured your BB Attachment Service is running because you stated other devices are able to open attachments.
Blackberry Attachment Service
To install the attachment service, follow the instructions below:
Cradle your device. If you have a password enabled, you must enter it before your device will connect to the desktop.
Open the BlackBerry Desktop Manager, double-click Application Loader, and click Next.
If prompted, enter your password again, then click Next.
In the Application Loader Wizard window, make sure the box next to BlackBerry Attachment Service is checked. Click Next, then Finish, then Close.