Many applications can read or write in differnt formats, the banner is only telling you that a non-native Wordpad format might not be fully represented in Wordpad. It's normal.
Curiousity -> What is the file extension or filetype of the file that gives you the banner.
Try File Explorer search
Navigate to This PC
Copy the following line and paste it in the search box
Date:08/09/16 .. 08/10/16 kind:=Document
Click the Type column heading to sort by type, then let the search cook.
Look at the document types that Wordpad can save as
You might find it that way - best of luck ... I'm out of ideas