Yes, some mail softwares (gmail notably) will not be fooled by the extension trick.
Please use AxCrypt to encrypt it, and then key share it with ‘email@example.com’ and then send the encrypted file.
Or, upload it to dropbox, Google Drive or similar and then share it with ‘firstname.lastname@example.org’.
Also, that website… That does *not* look like http://www.axantum.com/ . This is what it looks like: