Files Manager: Clicking "Add files" does not do anything

I have recently installed “File Manager”. However when I click “Add Files”, it does not do anything. Screenshot attachedFile_manager_Add_files_does_not_work

Is there any log file where I can check if there is any error message?

Thanks in advance for your help.

Check the JavaScript console from the browser’s developer tools.

Thanks mflorea.

I can see following error message

The resource from “https://xwiki.sintelix.net/bin/download/FileManagerCode/TreeWidget/tree.min?rev=1.1” was blocked due to MIME type (“text/css”) mismatch (X-Content-Type-Options: nosniff).