Filter rows based on Where condition AND-OR ?

Try the below and show us the Print text value: (I could not test it)

SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO

-- =============================================
-- Author:              <Reddy>
-- Create date: <17/02/2014>
-- Description: <GET BS Attachments>
-- =============================================
ALTER PROCEDURE [dbo].[GetBSAttachments] @StartRowIndex AS  VARCHAR(10)
        ,@MaximumRows AS  VARCHAR(10)
        ,@AddendumNo AS NVARCHAR(254) = NULL
        ,@ClaimNo AS NVARCHAR(254) = NULL 
        ,@Description AS NVARCHAR(3000) = NULL   
        ,@fileType AS VARCHAR(12)
AS
BEGIN           

        DECLARE @sqlColumns AS VARCHAR(8000) = ''
         DECLARE @sqlCond AS VARCHAR(8000) = ''
        DECLARE @sqlCond1 AS VARCHAR(8000) = ''
        DECLARE @sqlLimit AS VARCHAR(500) = ''
         DECLARE @sqlCount AS VARCHAR(500) = ''

        Set @AddendumNo = (Select Case when LEN(LTRIM(RTRIM(@AddendumNo))) = 0 Then NULL Else @AddendumNo End)
        Set @Claimno = (Select Case when LEN(LTRIM(RTRIM(@Claimno))) = 0 Then NULL Else @Claimno End)
        Set @Description = (Select Case when LEN(LTRIM(RTRIM(@Description))) = 0 Then NULL Else @Description End)
        
        
        IF (@fileType = 'Attach')
         BEGIN
                SET @sqlCond1  = 'AND FileExtension <> ''msg'''
        END
         ELSE IF(@fileType = 'Email')
        BEGIN
                SET @sqlCond1  = 'AND FileExtension = ''msg'''
        END

        SET @sqlColumns = 'SELECT   * FROM
        (Select id,AddendumNo,Description,ClaimNo , ROW_NUMBER() OVER (ORDER BY id) AS RowRank '
        SET @sqlCount = 'Select COUNT(*) AS BSAttachTotalCount'
        SET @sqlCond = ' FROM EFMS.dbo.Table1 WHERE FileExtension IS NOT NULL ' + @sqlCond1 + '

        -- Have to modify this logic
        AND (AddendumNo=  ISNULL(''' + @AddendumNo + ''',AddendumNo))
        AND (Claimno=  ISNULL(''' + @Claimno + ''',Claimno))
        AND ((''' + @Description + ''' IS NOT NULL AND Description like ''%' + @Description + '%'')  OR (''' + @Description + ''' is null and Description=Description''))'
 
        SET @sqlLimit = ' ) AS BSAttachmentsWithRowNumbers WHERE RowRank BETWEEN ''' + CONVERT(NVARCHAR(10), @StartRowIndex) + ''' AND ''' + CONVERT(NVARCHAR(10), @MaximumRows) + ''''


        Print @sqlColumns + @sqlCond + @sqlLimit -- To show in the grid by page size     
        Print 'Another query'
        Print (@sqlCount + @sqlCond) -- To get total records count with selection criteria.
                 



END