Recommended includes do not include *.[lmn]df, which are extensions MS SQL Server normally uses. However, I think the SQL Server install somehow, somewhere adds mdf and ldf (though not ndf, I believe) to one of the lists with actual excludes. It might be in the Snapshot agent; it's not in the usual FilesNotToBackup registry key.
I have come across the list with exclude locations, it might be on this forum somewhere, but I can't find it right now.
If you have an issue with backups failing because of mdf and/or ldf files on a SQL Server system, exclude the database/log files and schedule a database dump in SQL Server, then backup the dumps. Or use the SQL TDP. Or both. You really don't want to backup <backup>.mdf; you want to backup a dump or the actual database (which you can do online).