Not a valid filte-type selected

For some time now (early December), applicants have been receiving the message “Not a valid file-type selected”. An application is no longer possible. I actually use the standard form. I have now created the form again manually in the settings, but the message still appears. I have tried both .docx and .pdf in my tests – neither worked. Any tips?

WordPress version 6.4.2
Plugin version 2.7.1

The page I need help with: [log in to see the link]