Saving Excel Files to Correct Format with Macro

I have a macro which copies a sheet to a new workbook and then saves the workbook and attaches it to an e-mail.

When I open the attachment in the e-mail, it shows the following error:

"the file that you are trying to open is in a different format than specified by the
file extension. Verify that the file is not corrupted and is from a trusted source before
opening the file. Do you want to open the file now?"

When I click "yes" to open the file, the file is fine. However, when the email is sent to customers, the error scares them a bit and a lot of them do not wish to open the file.

My code is as follows:

    Dim FileExtStr As String
    Dim FileFormatNum As Long
    Dim Sourcewb As Workbook
    Dim Destwb As Workbook
    Dim TempFilePath As String
    Dim TempFileName As String
    Dim OutApp As Object
    Dim OutMail As Object

    With Application
        .ScreenUpdating = False
        .EnableEvents = False
    End With

    Set Sourcewb = ActiveWorkbook
    
    Sheets("Custom").Copy
    
    Set Destwb = ActiveWorkbook


With Destwb
FileExtStr = ".xls"
End With

' 'Change all cells in the worksheet to values if you want
With Destwb.Sheets(1).UsedRange
.Cells.Copy
.Cells.PasteSpecial xlPasteValues
.Cells(1).Select
End With
Application.CutCopyMode = False

'Save the new workbook/Mail it/Delete it
TempFilePath = Environ$("temp") & "\"
TempFileName = "Confirmation Letter - Ref " & Ref & ""

 Set OutApp = CreateObject("Outlook.Application")
    OutApp.Session.LogOn
    Set OutMail = OutApp.CreateItem(0)
    
With Destwb
.SaveAs TempFilePath & TempFileName & FileExtStr

With OutMail

' Compose the message
Msg = EmailBody

            .To = "" & Email_To & ""
            .CC = ""
            .BCC = ""
            .Subject = "" & Email_Subject & ""
            .Body = Msg
            .Attachments.Add Destwb.FullName
            If Attachment1 = "" Then
            ' Do Nothing
            Else
            .Attachments.Add "c:\HBPOS\Letters\" & Attachment1.Value
            End If
            If Attachment2 = "" Then
            ' Do Nothing
            Else
            .Attachments.Add "c:\HBPOS\Letters\" & Attachment2.Value
            End If
            If Attachment3 = "" Then
            ' Do Nothing
            Else
            .Attachments.Add "c:\HBPOS\Letters\" & Attachment3.Value
            End If
            If Attachment4 = "" Then
            ' Do Nothing
            Else
            .Attachments.Add "c:\HBPOS\Letters\" & Attachment4.Value
            End If
            If Attachment5 = "" Then
            ' Do Nothing
            Else
            .Attachments.Add "c:\HBPOS\Letters\" & Attachment5.Value
            End If
            If Attachment6 = "" Then
            ' Do Nothing
            Else
            .Attachments.Add "c:\HBPOS\Letters\" & Attachment6.Value
            End If
            .Send
        End With
.Close SaveChanges:=False
End With

'Delete the file you have send
Kill TempFilePath & TempFileName & FileExtStr

Thanks in advance