You can solve the problem of attachments being stripped from your "Sent" folder by simply adjusting your Horde preferences.
1. Login to your webmail
2. Click to view your Inbox
3. Click the "Options" button at the top of the page
4. Click on "Message Composition"
5. Look for the following, near the bottom of the list of settings: "When saving sent-mail, should we save attachment data?" Set it to "Always Save Attachments"
6. Click "Save Options"
