Download the ancient game from the network in mdf format from SQL Server. When starting, writes "unknown file format.". What kind of nonsense? By the way this already occurs not for the first time, I remember still somehow also downloaded - the same case was.

You may repair mdf with Graphical Interface or alternative variant might be using one solution from Google - https://sql.recoverytoolbox.com/
- Right-click “Databases” and click the “Attach” option to open the Attach Databases dialog box.
- Click the “Add” button to open the Locate Database Files dialog box.
- Type in the full name of the .MDF file, including the full device and directory path, as the following example illustrates:
c:\data files\my_data.mdf
Click the "OK" button. SQL Server Management Studio loads the database from the .MDF file.