Fix to sending test emails.

[tujko]

Fix to getting "MIME type ('text/plain') is not executable" error on the configuration page in admin and tort beeing able to send test emails.

Description (*)

You aren't able to send test emails in the module configuration and for the testemail.js file you get the following error:
because its MIME type ('text/plain') is not executable, and strict MIME type checking is enabled.
When you click the Test Now before the fix nothing happens.

Manual testing scenarios (*)

1. Go to Mageplaza->SMTP->Configuration
2. Configure extension and save.
3. On the page go to SMTP Configuration Options->Send Test Email, fill in Send To field and click Test Now.

Questions or comments