My current workaround is MsgBox(Mid(IO.Path.GetExtension(attachment.FileName), 2))
Thank you for writing to Aspose support team.
I would like to share the working details of attachment extension in this case. MapiMessage takes the Extension property value from the
PR_ATTACH_EXTENSION mapi property. If this property is absent, the Extension
property gets nothing. You can use the following sample code to determine if this property exists or not. Please let us know if we can be of any additional help to you in this regard.
MapiMessage mapi = MapiMessage.FromMailMessage(“AEMsgWithAtt.msg”);
foreach (MapiAttachment att in mapi.Attachments)