I have the same problem. I use WinHTTrack v.3.43-5. 
In Option panel : MIME Types
I tried 2 times, with the following MIME associations:
First:
File type: asp MIME identity: text/html 
The second time:
File type: asp MIME identity: text/html 
AND
File type: asp MIME identity: application/x-html 

But in both cases files were saved as .asp
Why?