why oh why did I think this was a trick question?:)

Of course .ldf is a sql server file extension... but is .ldf really a "sql server database" file extension. My reasoning? For a fully functioning sql server database you need all the .mdf and .ndf files that were set up when the database was created... the .ldf is merely a list of recent transactions. Lose either the .mdf and .ndf files and you will have nothing that will work... delete the .ldf and you simply lose a few transactions.

One valuable point missed I think!