Is it possible for an attachment to be added to a received email?

Hi

Is it possible for an attachment to be added to a received email?

Because I thought not, but that's what I'm seeing

I can see on our gateway that an email came in with one attachment on it, and yet that same email is sitting in someone's inbox with other, completely unrelated files attached to it, that it's impossible for the sender to have access to.

Is it possible these files have been attached after the email reached the users inbox?

Both Outlook 2010 and OWA show the same attachments

Exchange 2010 SP2

Thanks