You won't be able to do this automatically any way -
Someone will send an attachment, and the other person will reply with a revised version of the attachment with the same document title and all.
What if a person adds data to the file? then the file is no longer identical its bigger. Or if its removed data? then the file is smaller? What if someone wants the file from 2 revisions ago? Opppsss this file is now gone as we deleted it based on date...
Really the only way to perform cleanup like this is have people look into the chain emails and delete whats not needed.
The one thing we ALWAYS encourage is use of a file share put the file on the file share and have people email out LINKS. The link takes up no room and it hits everyone to one central location but if this is between internal and external people that will not work either.