have you tried opening it in Microstation?
Years ago (back when 8 character filenames were still the limit) some outfits would change the extension of their DGN files to something else to signify discipline or other things. Microstation would still open the files, regardless of the extension.
This may be one of those cases, otherwise I'd suggest googling it, theres a few sites that catalog "standard" filetypes... they may be able to help.
HTH