If you don't specify the body of the message in the command line, mutt will try to open your text editor so that you can enter it. If you specifically want to send an empty message, you can do it this way:
mutt -s "files[CONFIDENTIAL]" -a files.zip < /dev/null -- firstname.lastname@example.org
If it still shows a verification message, the problem may come from your .muttrc, and you may try the following:
mutt -s "files[CONFIDENTIAL]" -a files.zip -F /dev/null < /dev/null -- email@example.com
Unless your .muttrc defines a specific way of sending e-mail, for example if it contains something like:
set sendmail = "/usr/bin/msmtp -a you"
In this case, you may copy the interesting lines (all options that you want to use, such as
set crypt_autoencrypt if your files are confidential) of your .muttrc into a separate configuration file, and replace
-F /dev/null by
Hope that helps!