SQL 2012 Autogrow and file extension problem
Can the extensions be changed back without any conflict?


Not while the database is active, but you can offline it, rename the files, then bring it back online:


Check the virtual filenames as you'll need to refer to them in a later step:
SELECT name, physical_name, type_desc FROM my_db.sys.database_files
Offline the database:
ALTER DATABASE my_db SET OFFLINE WITH ROLLBACK IMMEDIATE;
(note: the above will kick out other connections and active transactions, if you want to be a little safer to other potential users remove the "WITH ROLLBACK IMMEDIATE" and it will instead fail if there are other open connections)
Rename the files in the filesystem
Let SQL server know what you have done:
ALTER DATABASE my_db MODIFY FILE (NAME='data_file_name', FILENAME='d:\dbs\data\my_db.mdf');
ALTER DATABASE my_db MODIFY FILE (NAME='log_file_name', FILENAME='d:\dbs\data\my_db.ldf');
Bring the database back online:
ALTER DATABASE my_db SET ONLINE;



  I was wondering if anyone has seen this before


I've seen it. It is most likely that someone created the database by hand (rather than using SSMS or other tools), specified file locations (or just names) different to the defaults, but got the filenames slightly wrong. SQL Server would not have complained at all because is doesn't actually care what the filenames are, they could be named "B12746289863413584F2EFC7A9A35815" and "D040839B958143C890A760B5A489E944" and SQL Server would be just as happy to use them as instructed, the mdf/ndf/ldf and having meaningful names generally conventions are just to give us humans a visual clue as to the purpose of a given file.


  what the best way to resolve it is.


As it doesn't upset SQL Server I would be tempted to say "just leave it alone" for production databases. For databases used for development/QA/UAT/other then either the above or alternately detach and re-attach the database instead of offlining it.