This fix appeared elsewhere in another thread on the Apple Support Community:
1 - Create a rule in the Mail Preferences pane
2 - Call it "Get My Paperclip Icons Back!"
3 - Set conditions as below :
A - If ALL of the following conditions are met :
B - From Ends With [Your email address here] (you can do this for multiple too]
C - Any Attachment Name Contain "." [with the quotations around the dot]
Perform the following actions :
D - Mark as Read [which really makes no impact on how its viewed]
Click Ok. When message "Do you want to apply your rules to messages in selected mailboxes?" I chose "Dont Apply". [But you can click "Apply" if you are in the "Sent Folder"
Then go to your sent folder, Select your messages [recent/all], and right click "Apply Rules" or from the menu Message -> Apply Rules.
Voilla! Paperclip icons are back. Plus "Mark as Read" did no harm to your already colored/flagged sent emails.