As of right now if a user is using a web browser to check their email and they get a PGP/MIME encrypted message there is no way for them to break up the MIME message after it is decrypted from the command line. The addition of a package like mpack would resolve this.
Obviously a better solution is for the user to configure a mail client that supports this through a GUI, or the sender can not use PGP/MIME but instead encrypt each attachment individually, or the user can `apt-get install mpack` whenever they need to use it.