In order to clear your SMS attachments from your phone, you need to launch the application and connect your device to your computer. Right click on your phone icon and select Backup now option. Select a destination folder and click OK. After the process is finished, right click the backup and select Clean. The first tab from the pop-up window contains your message attachments. You can choose to keep some attachment files by selecting them and clicking the Export button. Click the OK button to begin the cleaning process and after that right click on the backup and select Restore. Check the Remove items from device that will not be restored box and press OK. For further information, you can check the official web page.