give use interface for creating body of mail

Hello imran khan

 You are having the answer with you only. What all you will be needing is to use one multiline textbox which gets \n\r when user press enter key that is Envirnoment.NewLine 
Also for attaching files you have to use the FileUpload control in asp.net 2.0 .

 If FileUpload1.HasFile Then
            Try
                FileUpload1.SaveAs("C:\Uploads\" & _
                   FileUpload1.FileName)
                Label1.Text = "File name: " & _
                   FileUpload1.PostedFile.FileName & "<br>" & _
                   "File Size: " & _
                   FileUpload1.PostedFile.ContentLength & " kb<br>" & _
                   "Content type: " & _
                   FileUpload1.PostedFile.ContentType
            Catch ex As Exception
                Label1.Text = "ERROR: " & ex.Message.ToString()
            End Try
        Else
            Label1.Text = "You have not specified a file."
        End If.

Please refer this link for more
http://msdn.microsoft.com/en-us/library/aa479405.aspx#uploadasp2_topic2

Happy Coding takecare