You can use RESTORE FILELISTONLY to see if it's a backup.

>Good point, Sergey. I didn't think it was a backup as it had a .DAT extension. I tried restoring it and it seems to have worked!
>
>
>>You cannot attach SQL Server backup, you've to RESTORE it.
>>
>>>
>>>I asked a client to send me a backup of their SQL Server database. I received a .DAT file! Now, I'm not sure what I can do with this. I tried Attaching it to my SQL Server, but got the message that this is not a primary database file.
>>>
>>>Is there anything I can do with this file? Or do I need to tell them to do a backup using the SQL Server Management Studio (or whatever is the equivalent for the version of SQL Server they have)?