Is there a reason the Primary filegroup is not created using the .mdf extension?
While SQL Server does not require a specific file extension, .mdf is the standard and most commonly used extension for primary database files. Using .mdf would align with Microsoft SQL Server conventions and match the default behavior when creating a database through SQL Server Management Studio (SSMS).
Adopting the .mdf extension for the Primary filegroup could provide several benefits:
Improves consistency with SQL Server best practices and administrator expectations.
Reduces confusion when identifying database files during maintenance, troubleshooting, migrations, or backups.
Aligns generated databases with the default file naming conventions used by SSMS and other SQL Server tools.
Makes it easier for DBAs and support teams to quickly distinguish primary, secondary (.ndf), and log (.ldf) files.
Although this is primarily a naming convention rather than a functional requirement, standardizing on .mdf for the primary data file would improve usability and consistency across environments.
https://stackoverflow.com/questions/1175882/what-is-an-mdf-file