Piecemeal restore, filegroup and file extension
File extensions are meaningless to the application.  For example, create a text file and name it textfile.foo and open it in Notepad.  Notepad doesn't care what the filename is--it just opens it because you told it to open it.

SQL Server works the same way.  You can name the files with any extension you want and it doesn't change how SQL Server uses them.  The first data file, which is typically the one with the .mdf file, will always hold the metadata and can never be removed.  The metadata will not be spread out to other files just because you gave them an .mdf file extension.

File extensions are almost always for the humans--so that we can more easily keep track of things.  Accordingly, we use the .mdf extension on the first data file and .ndf on additional data files so that humans will be able to easily tell which file contains the metadata and can never be removed from the database.

I'm sure there are some applications out there that require file extensions to meet certain requirements, but SQL Server isn't one of them, at least not in regard to the file extension on database files.