After struggling to figure out what I would do since Dropbox was now saying that one of my very essential files could no longer be opened on my Android phone because this Word document was encrypted in Microsoft Word, I accidentally discovered that the problem can easily be solved if you simply ignore the message "File type not supported" and click on the vertical three-dot-menu at the top right of the screen and then select "Open with..." Then choose Word to open the document and all is exactly as it used to be. The message "File type not supported" should definitely be changed to avoid this confusions.