When Outlook 2007 or Outlook 2010 is running and installed side-by-side with Outlook 2013, Outlook shows the following message when you click Send To > Mail Recipient in Windows Explorer or click File > Send As Attachment in another program: This action is not supported while an older version of Outlook is running

In order to move ahead, you can try the following workaround: First, close the earlier version of Outlook, and repeat your steps. Or create a new email message in Outlook and click Insert > Attach File to attach the file. Viola, it just works!!