pass a string variable in an sql query executed using EXEC sp_executesql

how do I can pass a string variable in an sql query to be excuted using EXEC sp_executesql?

the original and working query is:

update database01.dbo.Docs SET property1 = NULL where [name] like 'doc.xls'

I need to build this query to look like:

DECLARE @FileName   NVARCHAR(30),
        @Query      NVARCHAR(MAX)
SET @fileName = 'doc.xls'
SET @Query = 'update database01.dbo.Docs SET property1 = NULL where [LeafName] like ' + @FileName
EXEC sp_executesql @Query

Many thanks for support.

Regards