MacOS – Mac Mail application removes attachment when forwarding mails

I believe this is in:

~/Library/Mail/V2/
  IMAP-[address]@[server]/[Mailbox].mbox/
  (various folders)/Attachments/(various folders)

If you look in your Library folder for the Mail folder, then explore that a bit, you can hopefully see what I mean. To see your Library folder, in the Finder, open the Go menu, hold opt or alt (depending upon how your keys are labelled) and then choose the Library option that appears.

Be careful not to change files in the Library folder unless you understand the consequences and have a good backup...

I do not know for the OS X 10.6.8 but on 10.9.5 it works like this

Create new Smart Mailbox

Once it is done finding all emails with attachments

Select all or specific one

Under the Messages tab select "Remove Attachment".

Or

Create a keyboard shortcut for it

System preferences Keyboard - Shortcuts tab

Select App shortcut

Click on (+) sign)

Select Mail in the Application window

Write "Remove Attachments"

Now choose your favorite keyboard combination for it.