Error 5171, cant attach data base

Hello,

I'm trying to attach a backup database on sql saved as .bak. 

I get the following message:

TITLE: Microsoft SQL Server Management Studio
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Failed to retrieve data for this request. (Microsoft.SqlServer.Management.Sdk.Sfc)

For help, click: http://go.microsoft.com/fwlink?ProdName=Microsoft%20SQL%20Server&LinkId=20476

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ADDITIONAL INFORMATION:

An exception occurred while executing a Transact-SQL statement or batch. (Microsoft.SqlServer.ConnectionInfo)

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C:\Users\Public\db\Northwind.bak is not a primary database file. (Microsoft SQL Server, Error: 5171)

For help, click: http://go.microsoft.com/fwlink?ProdName=Microsoft%20SQL%20Server&ProdVer=11.00.2218&EvtSrc=MSSQLServer&EvtID=5171&LinkId=20476

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BUTTONS:

OK
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what should I do?