The right function only reads the last four characters. The last four characters of the xlsx file are xlsx and not .xls
It is easy enough to demonstrate. Select a message and run the test macro below

Sub Test()
Dim olMsg As MailItem
    On Error Resume Next
    Set olMsg = ActiveExplorer.Selection.Item(1)
    CheckAttachments olMsg
lbl_Exit:
    Exit Sub
End Sub


Sub CheckAttachments(olItem As MailItem)
Dim objAtt As Attachment
    For Each objAtt In olItem.Attachments
        MsgBox Right(UCase(objAtt.fileName), 4)
        If Right(UCase(objAtt.fileName), 4) = ".XLS" Then
            MsgBox objAtt.fileName
        End If
    Next objAtt
lbl_Exit:
    set objAtt = Nothing
    Exit Sub
End Sub