Encrypting a file and how to decrypt without using extension
How do I send an encrypted file to someone who doesn't have the kodamail extension?

This can be done using AES 256 encryption, you just have to figure out a secure way
to get the pass phrase to them so they can decrypt it.

See Encrypting a file using either Public Key (PGP) or AES 256 encryption
and follow the instructions for AES 256 encryption.
Attach the file to your email and send. The receiver of the email downloads and saves the encrypted file, then goes to:
Decrypt AES 256 files. They select the file and are prompted for the pass phrase, the file is then decrypted and placed in their downloads folder.

When you give the receiver the pass phrase, also send them the link (Decrypt AES 256 files) to the decryption page.