How can I configure Mutt's GPG support to use simple ASCII armor instead of PGP/MIME?

Question

Since I started using signed (and sometimes encrypted) mail with PGP/MIME, I've heard about messages being marked as spam, making the communication less reliable. I found that this wasn't the case if I wrote the email outside of mutt, signed it with gpg --armor and pasted the result into mutt. Additionally, I also consider the armor to be more portable. After many Duck Duck Go queries, I've been unable to find the answer. How can I make mutt use ASCII-armor instead of .*/MIME?

Here's the relevant part of my muttrc (i. e. everything that starts with pgp or crypt):

set pgp_autosign                = yes                                          
set pgp_replyencrypt            = yes                                          
set pgp_self_encrypt            = yes                                          
set pgp_sign_as                 = ...     
set pgp_self_encrypt_as         = ...     
set pgp_timeout                 = 120                                          
set crypt_opportunistic_encrypt = yes