Just add a simple condition e.g.
Public Sub AIG(MItem As Outlook.MailItem)
Dim oAttachment As Outlook.Attachment
Dim sSaveFolder As String
Dim strText As String
strText = "Text to find"
sSaveFolder = "C:\Test\"
For Each oAttachment In MItem.Attachments
If InStr(1, oAttachment.fileName, strText) > 0 Then
oAttachment.SaveAsFile sSaveFolder & oAttachment.fileName
End If
Next oAttachment
Set oAttachment = Nothing
End Sub
Note that the path must end in a folder separator character and it is probably better to search the filename than the displayname.