Small fixes for the recent WP MailPoet module

Correct casing in the title
Anchor the use of ::File
Force body.to_s since it can be nil in corner cases
Land #3493, small fixes for WP MailPoet module
Add EOF newline to satisfy msftidy
Updated Mailpoet exploit to work with another version
Land #3494, @Firefart's update for Mailpoet's exploit