Your last update of the plugin (from today) solve the problem.
But it generates two others bugs :
– PHP Fatal error: Call to undefined function mime_content_type() in plugins\\mailgun\\includes\\wp-mail.php on line 249
– The name of the “email sender” is now “WordPress”, even if I try to change it with the filter “wp_mail_from_name” (it was working before today).
Please can you correct that ?