I assume that you are running this code from another Office application? If not you don't need to create an Outlook application as yolu are already in one.
You will need a function to identify the latest xml file in the folder, and if you want to retain your signature (and not running the code from Outlook) you will need the code indicated at the top of the macro to start Outlook properly. The code will pick the last modified xml file from your downloads folder.

Option Explicit

Private Sub MAIL_Click()
'Requires the code from http://www.rondebruin.nl/win/s1/outlook/openclose.htm
'Graham Mayor - http://www.gmayor.com - Last updated - 21 Jun 2018
Dim OApp As Object
Dim OMail As Object
Dim olInsp As Object
Dim wdDoc As Object
Dim oRng As Object
Dim signature As String
Dim strPath As String
Dim strAttach As String

    strPath = Environ("USERPROFILE") & Chr(92) & "Downloads\"
    strAttach = strPath & GetMostRecentFile(strPath, "xml")
    Set OApp = OutlookApp()
    Set OMail = OApp.CreateItem(0)
    With OMail
        .to = ""
        .Subject = ""
        .Display
        Set olInsp = .GetInspector
        Set wdDoc = olInsp.WordEditor
        Set oRng = wdDoc.Range
        oRng.Collapse 1
        oRng.Text = "FYI"
        .Attachments.Add strAttach
        '.Send
    End With
    Set OMail = Nothing
    Set OApp = Nothing
End Sub


Private Function GetMostRecentFile(strPath As String, strExt As String) As String
'Graham Mayor - http://www.gmayor.com - Last updated - 21 Jun 2018 
Dim fso As Object
Dim oFile As Object
Dim oFolder As Object
Dim strfilename As String
Dim dteDate As Date
Dim iPos As Integer

    On Error GoTo err_Handler
    strExt = Replace(strExt, Chr(46), "")
    Set fso = CreateObject("Scripting.FileSystemObject")
    Set oFolder = fso.GetFolder(strPath)
    dteDate = CDate("01/01/1900 01:01:01")
    For Each oFile In oFolder.Files
        iPos = InStrRev(oFile.Name, Chr(46)) + 1
        If Mid(LCase(oFile.Name), iPos) = LCase(strExt) Then
            If CDate(oFile.DateLastModified) > dteDate Then
                dteDate = CDate(oFile.DateLastModified)
                strfilename = oFile.Name
            End If
        End If
    Next oFile
    GetMostRecentFile = strfilename
lbl_Exit:
    Set fso = Nothing
    Set oFolder = Nothing
    Set oFile = Nothing
    Exit Function
err_Handler:
    Err.Clear
    GetMostRecentFile = ""
    GoTo lbl_Exit
End Function