Since the mainframe does not do Word in any form, exactly how do you think you are going to attach the Word document on the mainframe?
A simple text attachment can be done by SMTP code of
MAIL FROM: <MAINFRAME@xxxxxxxx.COM>
RCPT TO: <email@example.com>
Subject: Test attachment
Content-Disposition: attachment; filename=file.txt
followed by the desired data to be the attachment. Content-Type must match the actual data or there can be side effects -- binary data,for example, will not successfully be transmitted as text/plain.
If you want to find out the different attachment types, Google is your friend. Googling content-type
came back with 413,000,000 hits and some of them on the first page explain the whole MIME email concept quite well.