From a Support stand-point there is no way of achieving this in the product so it wouldn't be a Supported action to carry out. May I ask why you want to delete Attachment data?
My boss asked me to look into it.
You should check whether you are required to keep attachments (as with any documentation) for regulatory or auditting purposes. For example where I work, we're required to keep these for 18 months for SAS70 auditting, and, going forward, may need to keep these for a different time period should we need to adhere to Sarbanes Oxley auditting.
This is a decision through your internal processes, perhaps go back to your boss to find if he/she has a reason to keep/not keep attachments. Also consult with all your staff who use ServiceDesk as some may need files for longer than others.