Problem attaching mdf file in sql server 2008

I have an mdf file of sql server 2005 database now i want it to attach in sql server 2008 R2 but when i try to attach it, it gave me error saying.    Fail to retrieve data for this request.  An exception occur while executing a Transact-SQL statement or batch.(Microsoft.sqlServer.connectionInfo)
C:\program files\Microsoft SQL Server\MSSQL10_50.MSSQLSERVER\MSSQL\DATA\file6.mdf is not a primary database file. (Microsoft SQL Server, Error:5171) what can be the problem and how to fix it? Is this folder permission error or sql server 2008 have something missing