Solved - I have the same problem and found a solution to resurrect the clip icons in the sent folder.
I read it on another Apple post and revised it slightly to work a bit easier. See below :
[ Note : For the original solution click on the link and go down to oakmontoz's solution. https://discussions.apple.com/thread/3308790?start=0&tstart=0 ]
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.
Hope this helps!
Note : If you could make this rule apply automatically when you send mail, well... until Apple fixes this problem, this would be the seamless solution. Until then, you would have to do it manually.