Thanks for the answer Vessy.
I'm testing this by using the same document saved both on .doc and .docx extensions. It has a simple table and text in it as you can see from the attached image.
.docx one uploads successfully but I get an error "There was an error during the import operation. Try simplifying the content." on the page I mentioned on my first post.
And the error on debug level is "The Document Processing Library fails to convert the input file stream to RadFlowDocument"
But it's just a simple doc document.