The ideal way to handle this is for the web interface to detect if an attachment is an email and give you a "view" option that opens the email in a popup like a normal email would
This seems like it would be incredibly easy to implement
Is attachment dotType eml?
If yes, process attachment
Is attachment actually an email?
If yes, UI/UX show "view"
this could be expanded to display PDF's in the browser if the users plugin allowed for it, etc
I know its one of those, if we do one, how far do we go, but:
...would cover a good %95+ of it