Answer by grg for Is there a better way to manage Text Messages?

If you are jailbroken, you can use iCleaner:

^{Source: http://www.iclarified.com/images/news/37323/160771/160771.jpg}

Message attachments: it deletes files sent and received via iMessage or MMS. The “smart” setting only deletes files that do not show up in any message, while “on” deletes all attachments (messages with attachments will show a blank icon).

^{Source: http://www.funkyspacemonkey.com/icleaner-ultimate-ios-device-cleaning-app}