Thank you for the post.
From the article you linked above:
"7.2. Web users who use Internet Explorer to access a FileMaker WebDirect solution cannot drag and drop files into container fields. These web users may see the error message “Please select a valid file to upload.” Web users who use Internet Explorer should use the “Insert > Insert Into Container” menu item to insert files into container fields."
This is a limitation of WebDirect when using Internet Explorer specifically. I tested drag and drop into an interactive container with both Safari and Chrome and the file inserts without error.
The workaround would be to use one of the other tested browsers.