After sending an e-mail to someone, can the receipient of this e-mail change the file that was originally attached to a different file?
closed as off topic by Eight Days of Malaise, phwd♦ Mar 27 '12 at 21:01
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Yes, they can change the file to anything they want to. You cannot force them to forward a specific file during a forward.
The reason for this is due to you losing control of the content sent out when they're creating their own email.
The only way to mitigate this is to use a resource online that you control and not depend on email attachments.
My Suggestions - Dropbox Public/shared folder.
The public folder wouldn't be good for sensitive files, but the shared folder can be locked down to only people you invite to view the folder/file via email (they'd have to have a dropbox account as well, which is free).