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How to Change the File Extension for a Data File
In general, the file extensions that we use for SQL Server data files are MDF (Primary Data File), NDF (Secondary Data File - this is optional), and LDF (Transaction Log File). SQL Server works fine with any file extension but Microsoft recommends we use MDF, NDF and LDF.
But, a different file extension also work with SQL Server.
This article helps in finding a way to rename the file extensions.