The .mht format is an archived copy of a web page, it contains all of the images and scripts, etc, and can be dangerous when opened as these would be executed. It is also a bit strange when used in emails.
Do you know the source of this email and why they would attach an mht file (?), if either of these answers is no or don't know, then you shouldn't open this attachment anyway. Essentially avast is saying it is suspicious not infected as such.
If you can answer yes to both (and it is easy to fake the source, from address) and you accept the associated risk, you can modify the Mail Shield, Expert Settings, Mail Heuristics, Attachment check, but that would have an impact on all attachments.