I was sent a DGN file from Microstation by a DOT this week that would not open in TBC. I had @BrianVanPelt take a look at the file for me and when he opened it in Microstation they saw this header information in the opened file.
As you can see it says it is a DWG file but it has the DGN file extension. Microstation has the ability to use custom extensions for file names - in this case the file was saved with a DGN file extension but it is actually a DWG file - so a simple file rename makes it possible to open the file in TBC (Good Trick to Know if you get DGN files that will not open in TBC!).
Brian went on to say
“Going to bet the person from the DOT created an acad version without changing the extension.”
“Microstation has the ability to change the extension to something other than dgn or dwg.”
“*Our DOT (Iowa) uses: *.pho / *.dsn / .prf / and so on for file extensions.”
Useful to know and thanks Brian for your assist here - much appreciated