The extensions are just a naming convention, nothing more. A secondary data file is a secondary data file, whether it has the mdf extension, the ndf extension or any other extension that you like.
SQL doesn't enforce the file extension convention anywhere, it won't automatically make secondary files have a .ndf extension.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability