Gail is right. It's really difficult to know what to do if the source of the file is unknown.
Since it is a *.dat file, it's not likely to be a SQL Server Backup, but it could be since SQL does not "require" a backup extension for the file (even though it makes it easier
As suggested earlier, I would try opening the file in notepad or wordpad. A dat file might also be something from Excel for all we know. In any case, it is unlikely to be a database that someone just detached and forwarded along for you to attach to your server.
You might suggest to whomever sent you the file to try taking a backup of the database, assuming one exists, and send you that file to work with the data.