I sent a GPG encrypted message with attachment (also encrypted beforehand) to myself. This is it using Evolution Mail client. You need to know that Evolution Mail makes encryption in email much less cumbersome.
First Things First
In order to use GPG (the open source implementation of Pretty Good Privacy PGP) generate your key-pair. Your public key is uploaded to a public server accessible to the public and can be downloaded by anyone interested to send you an encrypted message. Your private key is stored locally, encrypted and secured by a passphrase. Your passphrase should be good enough to protect from attackers trying to break the encryption.
I use Seahorse to manage my passwords and keys. It's a GUI for the GPG program. Nevertheless, I still use the Terminal to encrypt files that I wish to attach to the email message.
How To Encrypt Files
There are many programs that you can use to encrypt files or an entire directory (or even an entire partition). I am just documenting how I do it if i want to attach it to an email message. Just open a terminal and change directory to the file to be encrypted.
$ cd /home/name/Document/anyfolder
$ gpg -h #if you want help with the text commands
$ gpg --encrypt Doc.name --recipient #you must have the recipient's public key to encrypt
A new file is created in .gpg format. Look for this file when you are ready to attach it in your email message.
In Evolution Mail, to compose a message just type: CNTRL+SHIFT+M. The add attachment button is at the bottom of the compose window. To sign and encrypt the message click on menu options>PGP Sign and options>PGP Encrypt. I have mine configured to sign all my outgoing mail. You can edit preferences by going to the menu edit>preferences. Click the General tab and tick on Digitally sign with PGP. Digitally signing your emails means you protect its integrity. If the message is intercepted and modified it will be flagged as such.