uninstall your kunena component, clear your administrator and site cache and install again.
I try kunena v.5.0.13, and when using attachement again, I get a new problem, that is 500 Internal Server Error.
I use PHP v.7.1, does it affect?
I have enabled mbstring.dll extension, but still like that.
for details of his error is in the picture.
*thanks for your response.
Please share Invitex 3.0.5!