Word has got more finickity in its expectations.It may still be able to open
the file, but may have difficulty with the extension.

So e.g.if the file is really a "delimited text file" with commas or tabs
between the fields and carriage returns/line feeds at the end of each
record, you could try changing the extension to .txt or .csv and see if Word
recognises the file. (If you can open and read the file in Notepad, the
chances are it is a delimited text file of this kind).