Rebecca - you would need access to the email headers to set the attachment mime-type. Outlook doesn’t support this this. So here is what you can do instead:
On your Integration Server, go to the IntegrationServer4\lib directory and edit the mime.types file. Add a line for your mime type (application/EDI) and assign a file extension (“edi” for example). Restart the server.
When you send your attachment, just make sure the file name has the correct extension (myFile.edi for instance).
Hope this helps,
Theo